Knowing nothing about heaters, I'm curious: why does electric heat make no sense? My intuition would be that an electric heater would require far fewer components (especially considering the battery is already in place), be less dangerous, and require less maintenance. What is the relative efficiency of electric heaters to propane heaters?
Electric heat via resisters is very simple, but very inefficient. (EDIT: Thermodynamically space-heaters are 100% efficient, since they always convert their source energy into heat. They are inefficient from a monetary point-of-view.)
You can do electric heat via heat pumps, but it requires more equipment and it doesn't work if it's too cold. You can get about a 4 to 5 advantage over plain resistance heat.
Gasoline engines lose a lot of energy to waste heat -- but sometimes that waste heat is exactly what you want.
> (EDIT: Thermodynamically space-heaters are 100% efficient, since they always convert their source energy into heat. They are inefficient from a monetary point-of-view.)
I don't think they "make no sense", and I'm not sure a propane heater is the answer, but an electric heater is going to require the use of battery power otherwise used for getting you from point A to point B.
