When my idea for TC use is stay in places where you can wear T-shirt and shorts, my construction experience says that starting point for cold OR HOT stay, is to seal the camper and add insulation.
Simple experience between sleeping in 20' ClassC v/s sleeping in 40' Bus conversion.
I used them both in California cold nights using 1500W electric heater.
When small ClassC could not hold the temperature- the much larger bus did.
Why? ClassC had 3/4" walls with fiberglass insulation, when bus was all-metal, but 2" walls were sprayed with semi-solid polyurethane foam.
During my camper repairs I discover lot of holes in it.
Tank heating duct had no return, so hot air was pumped via gaps around sewer drain straight into the woods.
Electric cord compartment was open straight under bathroom sink, so with cable slot open, cold air was blowing into your bathroom.
Check the seals around refrigerator as the compartment is cold as well.
Than I discover that my furnace blows the hot air straight from the burners into metal cover exposed to the outside. Meaning you are heating your parking pad as much as you heat interior. There was no space for adding flat insulation, but I sprayed polyurethane foam on the cover interior, put plastic foil over and screw it back on the furnace. This way foam expanded to the shape of furnace- giving me +- R4 insulation, what sure beats sheet metal "heat exchanger" manufacturer design.
Jacks plugs have hefty holes on the inner side, generator and battery compartments are build as heat exchangers as well.
Bottom line - couple cans of polyurethane foam and some styrofoam left from Christmas packages can very easy double camper heat efficiency.