Originally Posted by macman83
Could it also be that it's drawing warm air from the inside through the basement to help keep it from freezing?
Could be. It seems like there would be a supply duct to that area if that were the purpose. I have an "all weather package" sticker near the entry door that should, I think, mean I have tank heaters to prevent them from freezing. I have the switches in the cabinet, so that's my story and I'm sticking to it
I'm sure there are others more proficient than me with HVAC systems, but my understanding has been that an AC or forced air heating system is a closed system; no outside/unconditioned air is introduced because it lessens the efficiency. You re-heat or re-cool already conditioned air that is closer to the desired temperature.
My rig might be different than others, but there are openings galore in the basement area through what is essentially the coach floor and then open to the belly area between the frame members; toilet drain pipe to the black tank, drains to the grey tank and a large hole for an electrical bundle.
The flexible ducts that supply heated air are also way longer than needed, and they wrap around and over things and turn back on themselves when they could have been installed in a straight shot.
To get back to the OP's question, I haven't crawled in far enough to look at the FA unit to see if or how hard it would be to attach a dedicated return duct to the heater.