[TN road dog]

I agree the heat duct will not keep the rear of the coach from freezing.
I installed a typical bathroom exhaust fan in the basement next to the rear wall at the top of the basement ceiling. I connected a 3” flex duct to it and ran it towards the rear of the underbelly close to the fresh water tank. I run the fan continuously on cold nights circulating air even when the furnace has cycled off. The air is basically near coach temperature as it’s in close proximity to the furnace. I can and have in extreme conditions, run a oil filled heater beneath the exhaust fan intake. Please Don’t confuse the oil filled heater with a oil fired combustion heater.
This has kept our underbelly above freezing down to 17 degrees. Not sure about temperatures below that.
Also, one winter I used the oil filled heater in conjunction with the exhaust fan to keep the water system from freezing instead of winterizing the unit.