I have an O2 Holiday Rambler Neptune which has one of these "Back Seat Heaters" in the wet bay.
If you look at the plug in connection to the heater, on each side there are two larger wires that connect with each other. The middle connection has two wires running on the supply side, and one lighter white wire running from the heater side up into the heater. These middle wires are connected to the thermostat. I accidently pulled the two center wires from the supply side out and the light on my control panel would not come on, with temps down to 15F. I connected it up again last night and it works fine. I got to see the 12V "Backseat Heater" actually work. There is a thermostat on the heater I set to come on at around 30F. I also plugged around the hoses going thru the floor of the wet compartment with towels to prevent cold air from coming in from the floor. We are full timing and have about 120 feet of water hose, which froze up on us a few nights ago. It is now 100% insulated and hasnt froze since.