OK, I'm lost here. You have some decision making to do. I would reach deep into your head, your heart, and your pocketbook, let them all duke it out in a safe place, and see who wins. The result will help you define your heating and cooling setup.
If your wallet wins, just go to wally world and buy a 15 dollar space heater. Use your existing propane furnace to heat on the road and the space heater when you can be on-grid. Propane is going to win hands down off-grid.
A hydronic radiator is most likely not going to be any more efficient than the furnace.
Any phase-change system that draws 500 watts or less isn't going to put any more heat out than the 15 dollar heater on high setting. When outdoor temperature droops, so does heat pump efficiency. A shiny, new coleman rooftop heat pump unit retails for around (only) 500 dollars, and draws the same power as the 15 dollar unit from the pole. However, it belts out at least twice as many btu's when it runs. Energy consumption may or may not be an economic factor when parked in a campground. However, it is safe to assume a manufactured unit will work well for at least a few years. Not so with anything improvised or homebrew.
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