The premise: winter camping along our PacNW coast is normally snow-free but quite cool. We've tried three methods in our 9.5' slide-in camper.
1) OEM propane furnace. Works great, BUT: very inefficient and a battery hog. Good for short, quick warmups. Not so good for overnight heat when off the hookups. Good 'dry' heat.
2) Catalytic or "Buddy" style propane heaters. 'Wet' heat; a pure disaster, causing torrential condensation throughout the interior. Promotes mold growth under the mattress and behind the seat cushions. Competely unacceptable. Requires opening roof vents and side windows for ventilation to such an extent it impairs the heat purpose.
3) Our solution: "High Anxiety!" (This is for benefit of the worry-warts, scolds, catastrophe claimants, and other 'Chicken Little' screech hens.) I did a LOT of research on compact diesel air heaters, especially as reviewed on u-toob. Note: in a former life I owned and flew a Ryan Navion airplane that contained a gasoline air heater. It was safe and worked effectively and reliably.
I bought a diesel air heater on eBay for $148; installed it near the door, low down on the side of the settee, with the combustion intake/exhaust tubes running into the adjacent storage compartment on the camper side, vented to the outside via small cutouts in the compartment door. Inside, the heater recirculates interior air, just like the OEM propane furnace. A wall-mount LED control panel and DC power connections were easily done.
Conclusion: we had a cold snap here on the island when temps reached 22 deg F; I opened interior cupboard doors to circulate warmed air through the plumbing; the heater ran for several nights/days. Nothing froze. (We don't 'winterize' in this coast climate; just use heat as needed).
The wife loves this heater. It is less than one-quarter the battery drain of the OEM furnace, and burns less than a gallon of diesel in a 24-hour run. This DRY heat helps remove cooking and respiration condensation in the camper.
Okay. HYSTERIA: the diesel air heater is Chinese-built. Let the worry-warts and flame-throwers commence!