Hi wbwood,
In my class C I had trouble with fresh water line freeze ups while traveling because the propane furnace thermostat would be "fooled" by the dash heater.
I replaced the return air grill on the furnace with a fan. On my rv that circulates warm "living space" air under the cabinets and pushes some air along the furnace duct work. I chose to use a dual window fan unit which is controlled by a mechanical thermostat that lives beside the water pump next to the outdoor wall of the RV.
The fan unit draws just 23 watts and works better than a 1500 watt fan based heater which I tried using inside the cabinet.
