I'm assuming that the concern on propane usage for running the furnace would be for those camping where there are no hookups (electricity) provided.
But, if you winter camp at a Site that provides electric hookup, wouldn't some sort of electric space heaters enable you to conserve on your propane?
I'm speaking of the radiant type of space heaters, such as:
oil radiator heater
There are several different brands of these and there is no exposed heating coils or wires to worry about.
Depending on the size of trailer, I would think one would do an entire trailer. When I lived in South Florida, I bought one of these for a girl friend that lived on Ft. Lauderdale Beach. She lived in an older apartment building that didn't have central heat. Even in Ft. Lauderdale, the temperatures will drop into the 30's at night and only go up to the low 50's during the day, during their 3 weeks of winter. One of these did her apartment pretty well. The living room area, anyway. At night, she would unplug it and put it in her bedroom