I have not experienced -20 but I have bought two ceramic heaters for the basements and the water compartment, I do not trust the heater in the water bay, it seems like it works and then it don't. I get electricity from the back basement and run an electric line over to the water bay and plug the other in the receptacle. My coach stays outside and I have not had any problem but be sure you do not allow water in the ice maker solenoid, I unhook the line and blow all water out and do not use the ice maker. As stated in the other post make sure you have enough LP to keep the heaters going. We have a DFT with a bath and half, behind the commode in the half is a covered cabinet, when you look down in the cabinet you can see most of the water lines, that go tot he coach filter and allow the heat go down into the space. Good luck, I hope I have my garage built when next winter comes. One other thing, I have three remote temperature sending units, one in water bay, one in the water filter bay and one on the tank in the basement, let us know how things go.
2009 Dutch Aire 4317
2014 Ford Explorer