We haven't found any need for additional heat as of yet, as most of our teardroping is done from late spring up to early Autumn. Never in the freezing cold.
We have a three way fridge fitted in the galley. The fridge gives of a fair bit of heat , so much so, that we have insulated the area behind the fridge to stop too much heat getting into the cabin. Even with the insulation some heat does get through, and its great to warm your feet on the wall behind the fridge.
I had thought if we needed more heat, we could put a vent through to the area with a hit and miss vent to help control how much heat comes in.
The fridges have a flue for combustion gases, or can be used of the campground mains electric hookups. They also have 12v supply that is used to keep it cool whilst towing. It connects to the alternator on the tow vehicle so as not to drain your battery.
The fridges aren't cheep to buy new, but we have plenty of second hand units availlable from older caravans.
3 way caravan fridge
Just a thought for thouse with slightly larger tears or standies
We've also fitted a stainless steel dish above the heat exchanger on the fridge that we use as either a food warmer, or towel drier, depending on what we need at the time.
Cheers Dave
Dave