Bedlam.. It will depend on the BTU output of your furnace and of course your propane supply and battery capacity. In my case I run my small inverter genny anyway so battery longevity isn't a big issue.
My pup up has a standard sized furnace and while I don't remember the BTU output, it's plenty. I could run the inside temp up to I'd say 80 with no issues no matter what the outside temp is but we like it in the 60's max anyway, where we keep our house at as well.
Being northerners, guess the cold isn't a big issue. We dress accordingly and sleep accordingly.
Of course a pop up will loose heat faster than a hardside but considering the minimal wall thickness of a hardside and the lack of insulation (on most but not all), the transfer of heat from the occupied space to the outside isn't a factor we worry about.
So long as your heat source is sufficient and you have sufficient propane to run it, all is good.
