how to combine black and grey rv tanks

Crazy Easy Way To Combine RV Black and Grey Tank

The greatest benefit of installing a composting toilet is not having to deal with a black tank; but what do you do with that unused tank that’s taking up valuable space in the RV?
We thought about adding another freshwater tank, or removing the black tank and adding a larger grey water tank, or even adding a PVC pipe with a shut-off valve between the black and grey to combine the tanks; but the idea of me (Jason) removing or replacing anything of this magnitude in the RV sounds like a horrible idea.  So I gave up…until we were having dinner one night with our friends the RV Geeks and they recommended the most ridiculously simple, zero install idea of how we could combine the RV black and grey tanks.

Before attempting to combine your black and grey RV waste tanks It is important to understand the capacity of your specific RV tanks.

With Roy, our Fleetwood Excursion, we have a 90 gallon freshwater tank, a 60 gallon grey water tank and a 32 gallon black tank.  This means while we are combining the black and grey tank we cannot be connected to city water.  Why not?  With a full fresh water tank we barely have the capacity to store all that grey water, and one thing you do not want to do is overfill your grey or black tank!  So here’s how we handle this process:

90 gallons of water will last us 7+ days no problem.  We can safely assume we use 2 gallons per day for cooking and drinking.  This leaves us with 76 gallons of freshwater that will eventually become grey water.  So when our Grey tank shows full on the level meter I attach my Twist On Waste Valve (sometimes called “Gate Valve” or “Knife Valve”) and open the black tank then the grey tank to let water flow in between the 2 tanks.  If our grey tank was full it means there is now 30 gallons of grey water in both our black tank and our grey tank.  Since our black tank is now almost full (30 gallons of grey water in a 32 gallon tank) I shut both the black valve and the grey valve.  Do not attempt to remove the twist on waste valve as there will be grey water in the pipe between the twist on valve, the grey valve and the black valve.

how to combine black and grey rv tanks

Again, the order for adding grey water to the black tank is:

  • Attach the Twist On Waste Valve and make sure it’s closed
  • Open the Black tank waste valve by pulling the “black” handle
  • Open the Grey tank waste valve by pulling the “grey” handle
  • Let the water “auto level” between tanks
  • Close both the Black and Grey valves
  • Leave on Twist On Waste Valve until after you dump

Here are the exact items I purchased on Amazon to combine my RV dump tanks:

I’ve been using this technique for a few months now and it works perfectly.  I’ve asked the engineers and the service team at Fleetwood if this is safe and they confirmed there is no problem with combining the black and grey tank using these recommendations.  I have also confirmed with Fleetwood and Sea-Land (the Vaccu-Flush toilet mfr) what would happen if I overfill the black tankThere are 4 valves built into the motor for the vaccu-flush system that allow water, and waste, to transport from the toilet into the black tank.  It would take 17psi or greater to force those valves to open the incorrect way, their claim is “it’s almost impossible to get that sort of pressure built up inside the black tank”…the more likely issue with over filling the black tank is the water will travel up the vent tube and start flowing out the vent opening on the roof!  Now that would be a sight, YUCK!

Another very important order is the tank dumping process:

  • Attach the sewer hose
  • Open the Twist On Waste Valve
  • Open the black tank and let it empty completely
  • Shut the black tank valve
  • Open the grey tank and let it drain
  • You do NOT want to open both the black and grey at the same time as it could overfill the black tank (it’s not likely but just in case)

We use this technique while boondocking which helps extend our wild camping and it’s extremely helpful when staying at a campground without full hook-ups (like a state or national park, a Thousand Trails property, etc.).  It’s worked great for us and with a little testing I’m sure it will help extend the time between your visits to the dump station.

What’s your take on combining the black and grey RV tanks?  Are you willing to give it a shot?  Please share your thoughts and insight in the comments below.

Of course there is a lot of assuming and variables happening here (Can my fresh tank hold more than 90 gallons?  Can my grey tank be larger than 60 gallons?  What if the RV isn’t level?), it’s not science.  So please take this information and use it safely, contact your manufacturer as every RV, year and Model are different, and ask their opinion.  Make sure you test this process on a small scale first to make sure it works before going all in with a full 90 gallons!  And if you haven’t read our disclaimer you should.  Also I’m not sure of the legality, or safety, of combining your black and grey tank if you have a normal toilet (i.e. you’re putting toxic waste and toilet paper into the black tank) so I cannot recommend this option unless you have a composting toilet, or you don’t use your RV toilet.  Good Luck!

Famous for my "how-not-to" videos, and typically the man behind the camera, sometimes I’m forced to be here in the “spotlight”. When you see my face you’re probably reading something more technical than adventurous, but either way I do my best to tell it like it is and infuse my opinions into the commentary…after all this is a blog and not MSN.

Comments (60)

  • Victoria Carter

    I know this post is super old but, I’m hoping you can still answer my question! What did you do to your black water tank prior to this to make it ok to use as an additional grey water tank? We’ll be removing the toilet that came in the 1995 Firan Telstar that we just bought, and using a cassette toilet instead so we don’t need a black water tank. But it’s definitely been a black water tank for the last 25 years so I’m assuming we need to do something to treat it before we just start using it as a receptacle for waste water and disposing of that waste water accordingly… any advice would be greatly appreciated!

    • Curious Minion

      If you’re dumping the tanks at a dump station, you don’t need to do a thing since black and gray are going into the same receptacle. After a few times of filling the gray tank, the black will be well rinsed. If you are planning to dump it somewhere that is NOT a dump station, that’s not cool. You should never empty that large amount of gray water anywhere but a dump station.
      Curious Minion

  • Shane

    Why doesn’t anyone double their fresh water storage? Seems way more useful…

  • gary collins

    I have a question for y’all I have a 2004 HR 5th wheel with 3 waste tanks all 40 gallon. One black and two gray. They all Y into one dump outlet. All three are cable operated inside the enclosed underbelly with pull handles outside the 5th wheel with. The kitchen gally gray tank is open all the time(not sure if cable is broke). The black tank does not shut off all the way and all water drains into the drain pipes and the shower, bath vanity, washer tank shuts off the way it should. I have installed a valterra twist on shutoff valve on the outlet. I am terrified of a cable breaking on the black and one gray tank and not being able to dump. With the valterra valve on the outlet, can I leave all three pull handles open and let the 3 tanks fill like one tank. I dump very regular so never have to worry about black tank getting too full. I peaked up in the underbelly and all three tanks are exactly the same and all mounted at the same level. Looks like a ton of work and I thought of cutting the two grays and combining the two and adding a second dump outlet. At my age I am not sure I can tackle this job. Thks in advance

  • Cindy Shawver

    I have a bumper pull camper 2015. How hard is it to remove the black and grey tanks? I want to hook up to sewer line and put a household toilet in.

  • Mark Wood

    Can u use your blackwater tank as a fuel sell to run your generator or fridge on…. Making a biogas then pipe it to fridge.. Hmmmm. Oh no honey fridge is warming up! Can you please drop a load so the ice-cream stays cold…. Lol…… But why not?

    • jim scott

      “fuel sell” did you mean fuel cell? if so, NO way. It wouldn’t be a fuel cell, it would be a biogas composter.. Also because you won’t be making propane, you’ll be making methane. As the names suggest they are different gasses, which is to say they are different molecules. Both molecules their made up of different amounts of carbon and hydrogen atoms. Because of their different molecular make up they require different oxygen to fuel ratios to burn cleanly. If they don’t burn cleanly you will produce soot, which is amorphous carbon mostly made up of carbon atoms. This soot will build up and could block or clog the burner assy’ in the frig. causing your frig to stop working and your your ice cream would be the consistency of shit! 🙂

  • Angel

    Hi Jason,

    I have a Travel Trailer and both of my waste tanks are 44 gals, usually we pretty much gets filled less than 1/4 of the black when the gray is almost full. I was looking for an option to equalize them and gets more time when boondock. I was using the bucket method from gray to the toilet but that kills my back. Now I found this idea which sounds great, my only concern, it is safe? I mean, when you are using the black tank and you open those valves you are pretty much mixing both waters which Im kind of worry if have black water and stuff in the gray will cause a sanitary issue. Do this mod/tip only works for people like you that doesn’t use the black tank? or maybe because of the angle of the piping and the water pressure coming from your gray when you open the valve it doesn’t allow the black to come to the gray?

    Thanks for all of your videos, im subscribed to your channel and I enjoyed a lot, actually, with all respect, you have a beautiful wife, im her #1 fan! 🙂 Happy camping!

    • John A. Slayton

      This combining of the black and grey tanks only works for the wynns because they do not use the blackwater tank for the toilet. A composting toilet does not connect to the black water tank.If you are using your rv toilet you absolutely do not want to mix the two. That is a very good way to clogged plumbing.

  • Christie

    “We have a 90 gallon freshwater tank, a 60 gallon grey water tank and a 32 gallon black tank. This means while we are combining the black and grey tank we cannot be connected to city water.”

    I’m probably being really dense here, but I don’t understand why you can’t be connected to city water. I’m also an RV newbie, if it’s not obvious!

  • Jason this is so logical. As newbies we got a lesson from a 30 yr veteran RV guy, who instructed me to always dump at no more than 2/3 full in the black tank. I was purchasing the twist on valve when he gave the “dumping class”. He indicated just as you are doing of having the twist valve on, but closed and the opening the grey tank up to flow into the black tank, taking caution to just really fill the drain pipes. Then closing the Grey and opening the black tank. I guess to grease things up. From there open the twist valve and let the black start draining. Again gauging on the progress and practice shut the black valve and open the grey valve. This time you were to really let the black fill up, so after opening the black and watching the dilution take place you can see the force of the volume of grey really mixing things up. Again using your best judgment shuting the grey valve and letting the majority of the black water out. He indicated that you never want to completely dump the black because the tissue, if you use it… we live like we are overseas and use the trash can for the main purpose of not having paper machete issues at the bottom of our tank, you can run into blockages and things just become cement.

    This whole process takes more time, but I do feel that the black tank get a through rinse. We of course also have the back flow capable with the twist valve which is amazing and then I realized that our Keystone Challenger has a sewage flush built in the water station. One thing to note there is that having the city water and sewage flush hooked up on a Y as they come from the dealers might not be the best idea if a check valve were to malfunction there might be the chance of contamination.

    I loved your solution because I had planned on having the greys and black combined in our rig once we install the Cinderella Motion incinerating toilet, we too would have no need for the black tank. Your solution is exactly what we are doing now and costs nothing more. Which is even better.

    What did you use to close the drain to the black tank from inside the rig when you put the composting toilet in?
    Also did you have to vent it? The incinerating toilets have to and I am devising a way to accomplish that. I believe there is a stack already piped in for the same purpose with the vaco toilets just need to determine how to access it.

    Thanks for the great posts.

    • Hey guys just saw the link for the “red plug” yellow now.

      Going to see about that exhaust pipe, too.


  • Gerard Haubert

    Have you ever considered or seen anyone replacing a std rv toilet with a cassette toilet (Thedford makes a number of models). Not having access to a dump station requires ingenuity.

    • I have not. A composting toilet would seem to be a better option over a cassette toilet.

  • Aria Billings

    Jason and Nikki,
    Okay, RV newbie here. I just bought a 1988 Mallard Overland (34′ A class) and we plan on installing a compost toilet and doing all this next month! Now for my question: what is the Camco sewer cap with hose connection for and where does it go? Our sewage drain area looks nothing like these newer models. There are two valves, one for each tank, and that’s it. No water pump button, no hose connection area, and nothing is labeled. Trying. To figure all this out with an old RV is a lot harder than we thought! But thanks for all your videos, they’ve been so helpful! You’re my favorite RV bloggers!

  • illya

    I was thinking, most people haul water in a plastic tank hidden, Why not make it a fishtank? you still get a water tank except its a beautiful centerpiece type of thing instead of a hidden jug. It could be be fake , not necessarily with the right filtration and auto top off but seems like it would be pretty cool. Double use of anything is like double value. I know i could easily design one that would be more than safe for drinking. The only issue is center of gravity, which really is not that big of a deal in a big RV. sharing ideas.

    Thanks, illya

  • Bill Toth

    There is a product called Bio – Clean which is used to keep drain lines clean by using “bacteria action” to make the bad stuff disintegrate and not stick . The key to it not to rinse so the activity of the germs carries on after you dump. It is bio friendly I believe and will also keep the smell down. It is available on Amazon. Bill

  • Bill

    I have a grey and black water tanks. they drain into a t with a single gate valve . Is this OK ?

    • Vance

      I have the same thing on my 2016 Heartland Gateway 3750PT. The rear toilet/sink and kitchen sink all empty from one valve. I want to cut where they come together and have a separate valve for just the grey water. Have you made any modifications to your current setup?

  • Pam

    We’re refurbishing a 20 yo trailer to use for weekends and occasional weeks-long trips. Love the idea of the composting toilet and not having to do the yucky dump. Don’t know if our use will justify the cost – but your videos are very persuasive!
    My question: I noticed the small cap and hose connection on that large sewer cap and it made me wonder whether a smaller hose would work to empty the gray water. If we’re only dumping gray water, and not staying connected at a park, would it be easier to use a 3/4″ or 1″ hose to do the dump, rather than the larger sewer hose?

  • Larry

    I am planning on buying a new fifth wheel to live in. If I bought the composting toilet and installed it before I even used the black tank, then couldn’t I use the black tank for additional water storage? Seems like I could.

    • The black tanks are made from different materials so you wouldn’t want to use that as drinking water. Which is why we opted to use ours for additional grey water storage.

  • Mark

    Hi Jason. I like the idea . . . Bought the good ‘stuff’. What I noticed is that the black and gray meet at a T-valve. With a cap. What is preventing me from simply leaving the cap on and opening the gray to flow into the black? Without adding a waste valve.


  • So glad this technique is still working for you guys. We’ll bet that it’s coming in handy out at Burning Man this week! Hope you’re having a blast.

  • Stephen Cobb

    As I plan my bus to rv conversion one question that came up was how large the grey and fresh water tanks should be. I will be using a composting toilet with the urine collection piped onto the grey water tank. I don’t want to go too small and rum low, but at the same time too large adds more weight.
    What would your optimal tank size be?

  • Hello,

    Sorry ahead of time if I am asking dumb questions but here it goes….
    1. We currently have an 82 gal freshwater, 62, gal black tank & 65 gal gray tank. Since you can dump your enviro friendly gray water – wouldn’t it make sense to add a new fresh water tank to replace the black tank so we would have more boon docking fresh water capacity?
    2. We have buttons that we push to open and close our black and gray tanks- ?? does the Twist on Waste Valve work on all tanks? What do I need to look for? I think the tanks are enclosed and not easily accessible.
    3. Tank sensors- we were having problems with both black & gray tank sensors working properly. We now use the GEO method for our tanks and the black tank sensors are working great. The gray tank – not so much- it always reads 2/3 full. Any suggestions?

    Thanks for all your information!


    • Grey water is far from enviro friendly, especially in protected environments. Dumping grey water introduces alien microbes into the environment that aren’t always welcome and cause real damage. There is a reason why dumping grey water is illegal in most places.

  • Samantha

    In your post you suggest the following method for combining the tanks:
    Again, the order for adding grey water to the black tank is:
    Attach the Twist On Waste Valve and make sure it’s closed
    Open the Black tank waste valve by pulling the “black” handle
    Open the Grey tank waste valve by pulling the “grey” handle
    Let the water “auto level” between tanks
    Close both the Black and Grey valves
    Leave on Twist On Waste Valve until after you dump

    But if you are actively using your black tank (no composting toilet), wouldn’t you want to open your grey tank first, so it fills up the extra space, then the black? And it won’t cause sewage to rise up into the gray water tank at all?

    I have a trailer and I am doing roughly overnighters, which means I really don’t need to get a portable waste water tank. I just need to come up with methods to move grey water into black water tank to extend grey water allowance. One thought is to siphon dish water into bucket, then drain into toilet. I am purchasing water saving methods via your Amazon store and had just bought the Oxygenics shower head.

    I am new to this concept and a little antsy about it, but it seems to make sense.

      • Samantha

        Good to know. We do have an outside shower hose. Seems like it is becoming a regular thing to include those on the newer trailer. Hadn’t really thought about how handy that could be. Any suggestions for privacy shower products?

        Also…what do you do if you are at a campground, not boondocking? I’m sure they wouldn’t appreciate people just taking showers at the side of the RVs. Guess that’s where you need to use the public showers if you don’t want to take on grey water in the tanks. Hmmm?

  • I just did this little trick to my RV and dude, I love you! Thanks for the idea. Now we can extend having to call the honey bucket for a few more days. By the way, I didn’t see this little device in your store. Just a hint that I would have bought from you if it had been there. 🙂

  • Brian

    Hi Jason and Nikki. First time, long time and all that. In my real job I’m a “tank expert”. While I don’t deal with RV tanks specifically – most of the stuff I do is much larger polyethylene and fiberglass tanks – projects that I work on daily in my job are very much in line with what you describe in this informative post and video.

    I would like to add some of my thoughts to everything above. The key point that I want to get across is that it really does not matter how different the capacity of your two tanks are. The key to what you are accomplishing is that the tanks will balance to an equal horizontal level. If the elevation of the TOP of the black tank is equal to elevation of the TOP of the grey tank, there is absolutely no risk of a much smaller black tank overfilling. The liquid level of the black will simply equalize to the liquid level of the grey. Since both tanks are vented and the operating pressure inside of them is essentially atmospheric (minus any column head pressure – but that doesn’t really matter) I see NO RISK of the black tank overfilling. Further, I can’t really see any scenario where grey water being transferred over to the black tank would spill out of the vent pipe at the top of the tank. For this to happen, the liquid level of the grey tank would have to be to the roof too… and we all know that isn’t going to happen!

    For peace of mind I would suggest that you determine the distance from a fixed point to the top of each tank. The easiest reference point would be grade on a level pad. If the top of each tank is an equal distance from the fixed point, from a practical perspective, you would never be able to overfill just one of the tanks (you would either be overfilling both tanks or neither). Knowing this information removes the guess work from the equalization process.

    • Tracy W

      yup Brian, that’s what I was thinking but you explained it SOOO much better! 🙂 thank you

      Just gotta make sure the camper is level, and pay attention to the capacity indicators and all should be well.

      Now I just gotta contact the pro’s for that direct shunt of the urine to the black tank so we can bypass that little bottle. That would be so sweet!

      • Tracy, that is our next project too! If you get yours done before we do, please let us know how it goes.

  • Tracy W

    OMG!! I LOVE THIS! …..we are also going to look into doing the mod thing that directs urine from the Natures Head directly into the black tank, which literally would be perfect. I would love to not have to empty that little urine bottle all the time. Since it’s safe to “water” the ground with urine and bio-degradable grey water, would it be safe to combine the two prior to “watering”?

    I was happy with our decision to buy the Natures Head but then I was thinking what should we do with our black tank. This would solve that issue perfectly.

    If all that attachy thingy is doing is leveling the water between tanks, even though your black tank is smaller it shouldn’t have any “higher” level than your grey tank anyway, right? The height of the water would be level, but the volume would still be less in the smaller tank.

  • AJ

    Very smart solution…seems easy enough. Very cool way to gain more grey water storage and extend your off grid living. Probably one of the easiest mods I’ve seen you do. Nice job!

    Love y’all’s new coach.

  • We’ll be doing this with our vintage Airstream next year – remove the toilet and replace with a Nature’s Head, then combine the tanks. It makes so much sense.

  • Dominick Trentacosti

    I’ve been thinking of how to do this for a while since my wife and I dig the idea of a composting toilet and need more gray water storage.

    Thanks for the idea!


    • Terri

      Question: We are picking up our first coach this Saturday! yippee! View Profile: We have well water on our farm. Is it ok to fill our water tank with well water? Thanks so much.


  • paul van

    Very clever solution. Watching the video really helps clarify the printed directions. A+

  • Doug

    During LTVA season, I always combine my gray and black tanks to give me almost a month between dump station visits.

    Other great things about the Twist-On Waste Valve: if any of the RV’s gray or black valves don’t close completely for any reason, the twist-on valve saves you from a face full every time you remove the cap! Plus, ever having to repair/replace those valves, especially if they are hard to reach.

  • So about 3 hours ago we were just sitting down to eat at this nice restaurant, when DING!… e-mail alert! It’s Jason & Nikki’s post about their sewer system, just in time for dinner! LOL

    We were thrilled to be mentioned in the post and video. Thanks so much! Great explanation Jason, and so glad this technique worked out so well for you. Great that you did due diligence on it by calling Fleetwood, too. We would have just thrown that valve on there and probably had water shooting out the roof! lol (of course we would have edited that out, as usual) 😛

    By the way, the restaurant we were at this evening was Morgan’s Bistro in Harrison Hot Springs, BC…. the wonderful place we all ate when you were here. Talk about timing! We were literally talking about having been there with you when your post came in. We were there celebrating our 18th anniversary, which is today. We chatted with Morgan a bit, and broached the topic of bartering a website for her restaurant (“We work for food!”). The meal was just as great as last time.

    Hope you two are still having a blast out there. Miss you lots.

    Love ya,
    Peter & John

    • if you work out the barter on the website for food thing let me know so I can make sure to come visit before you consume it all! Happy anniversary and thanks for all the great tips! You guys are the best Geeky friends ever!

  • Dianne Worthington

    I’m still in the research stage & not fterribly amiliar with all this, so this may be a silly question, but you talk about it being OK to “sprinkle” grey water when boondocking to get rid of some. How do u get it to come out slowly enough to be able to do that? I didn’t think u could control it & it came out rapidly

    • bayrider

      I don’t know about sprinkling but it is perfectly ok to dump grey water into absorbent soil away from campsites or water sources. It’s nothing but a little soap and food particles and is not toxic or dangerous, it will be absorbed and digested by bacteria in the soil. Of course you need to use good judgement, it will smell a little, you don’t want to contaminate areas where people will come in close contact. We will often dump ours off a secluded roadside on our way in or out of camps. We live on a few acres and I often dump it on my outer front lawn at home where I park the RV, this in CA where it won’t rain for six months, no problems or smell at all.

  • Aye, this is how the previous owners of our coach did this. They installed a household flushing toilet and the black tank fills quickly. Very clever if you ask me. I don’t know about using the gray tank for black overflow though. We just dump it twice a week.

    • It’s the other way around – you use the black tank to store more grey water, not the grey tank for black.


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