Still thinking that the furnace is burning propane while that morning coffee brews on electric. So get 2 to 4 large batteries to run the space heater properly and then figure out a way to charge and maintain those batteries. Then let us know if it is worth it.
For camping in a camper shell I kinda equate that to tent camping. When the sun rises you just get up and start your day. No reason to cozy around in a heated tent.
Sorry if I am a bit off with my perception.