Crazy Easy Way To Combine RV Black and Grey Tank

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 tank:  There 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!