Steve, many of us who do a lot of boondocking use supplemental heat because the forced air furnace is a battery hog...not to mention a propane hog.
We follow the sun, so don't usually camp in areas where it freezes. However, even here in Quartzsite during the winter, temperatures overnight can get down below freezing. In those cases, we do turn on the forced air furnace for a few minutes when we get up in the morning (we never run any kind of heat overnight) just to heat the basement...but our main source of heat is our Kozy World infrared heater, which doesn't use any battery power and uses much less propane.
2014 Winnebago Aspect 27K
2011 Kia Soul