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.