Originally Posted by dsol
My trailer has the same three switches which are not identifiable either. I also assume they are for each holding tank based on many other folks assertions, but I have no idea and rarely hit freezing temps. I don’t like using if boondocking because heat generation generally requires a lot of energy and will drain batteries. Obviously, not an issue if plugged in.
I too wonder about the fresh tank in freezing situations but don’t have an answer. Agree with @Persistent, one of the more knowledgeable folks on this forum that residual heat from the furnace and trailer itself will help but no actual knowledge myself.
I have two sets of switches for my tank heaters. One set operates using 12 volts. The other set operates using 120 volts. My 120 volt switches do not run from inverter power and so cannot work while boondocking.
With only 200 amp hours of battery capacity, I never run the 12 volt switches. I pulled the fuses to prevent accidental battery discharge. My propane furnace keeps the belly area above 40 degrees so I don't need the tank heaters when boondocking.
I measured the temperature in the belly and found that with the furnace thermostat set to 60 degrees, the belly temperature is 40 degrees. That temperature holds as outside temperature drops. I have tested it down to 15 degrees outside temperature.