As an option to what Jim has said--altho the furnace vents may be covered with a slide, there is some room for air circulation. We have traveled in an '05 36TK3 in winter time several times and had the furnace on set to 40degrees. Seemed to work ok, idea was to keep the basement warm. May not be the best idea, but it worked for us. Keep in mind, the truck will probably not keep up with the furnace blower motor use on the batteries, and also propane use. Would not run the furnace any higher than minimum temp set.
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired