Heat transfer would seem to be the only anwer for RV heating. Circulating hot water thru insulated pipes. Where you get the hot water is the question. I had an a pickup truck camper outfitted for cold weather and used a Force 10 propane heater for boats. i tried a diesel drip heater of the same type but it blasted us with to much heat in a small space. The Force 10 was a wall mount and we used it for years with good results. In fact I still have it even though the camper is history. These days we want something bigger like an airstream. I did see very big AS around 40 foot with a wood stove once. It was being refurbished. Just hook it up to an outdoor wood boiler and you have all the heat you will ever need.