Typically there is one main bay that holds your black/grey/fresh tanks, your sewer connections, water pump, water filter, etc. This is the MOST important bay to keep warm. I purchased this tiny ceramic heater and leave it running in the bay during freezing temperatures. It pulls about 5 amps and keeps all my pipes warm (exterior temp 20 degrees, inside bay 50 degrees). Some people recommend hanging a work light in the bay: I tried this and the bulb melted the plastic bay, and my water still froze so I don’t recommend this option.
For more tips and info, check out this post/video: How To Prepare an RV for a Freezing Winter Adventure