I have seen people who build covered decks that are really mud rooms. In these Mud rooms they put the heaters, so it frees up space in other areas. The last one I saw, had the deck so long it covered both doors, massive heater with stone around it. Just use some computer fans which are 12v, use even the small 1.5 or 2 watt panels to power them. The excess power can be diverted to a small 12v deep cycle battery, I got mine from a used Electric Bike dealer. One could put an addition onto the back, even insulated Canvas would work, then you increase the mobility of the RV/Camper while creating extra room and a place for the larger heater.
Now I am working on two projects, well actually 3 projects.
1st was simple, I gutted an old Kerosene heater, put in a small catalytic propane heater. I am using the old cabinet from the Kerosene in order to make it safer as well as direct the heat better. Last night, I put an old brake rotor on top, since it is solid metal it took a while for it to heat up, likewise it will take a while to cool down. So there is some mass.
2nd my mechanic uses the old oil he takes out of vehicles to heat his shop. He puts them in some soup cans, 3/4 full then squirts a bit of BBQ lighter fluid into them to get them going. After a while, he will put some wood on top, to really get it heated up. Heating cost $0, although you need to clean the chimney more. I have two portable camping wood stoves now. One brand new the other cost $40 with the home-made chimney.
3rd Will be making a rocket mass heater for this coming Winter and Fall, enough of spending so much money on Propane. I will of course use the propane as an emergency supply or a quick warm up, while the rocket mass heater is doing its thing.
Now seeing as this is a vehicle, RV there is not as much room in it as I would like, so the outlet pipes will be modified. I have picked up some used (although maybe never used at all) gas heater pipes. This will be for the outlet of the Rocket Mass Heater. Since it is already a duel pipe system, the inner pipe, the smaller one will be for the exhaust. The outer ring will be filled with Vermiculite or Perlite. (almost the same, slightly different but very similar) They both hold and retain the heat, are both very lightweight.
Next I will attach a cheap copper or other metal pipe to it, this will be the outside air, so it is warmed as it travels along to the Rocket Heater. The heat and fire will draw the air so no need for a fan for the fresh air intake pipe.
Next, since I finally found a local supplier of Clay, I can now make my Portable Rocket Mass Heater. The base will be a concrete slab/patio stone with some clay bricks with air gaps, so the floor does not get too hot. The combustion chamber will be a Propane Bottle, modified of course. Inside and outside will be the clay to absorb the heat. I will be using a 20pndr for size reasons. Since the Rocket Mass Heater is so efficient, picking up small wood from building sites and other small wood sources will provide the bulk of the fuel. In this I might experiment using the used oil as well. There is a few hundred gallons kicking around this place and I already have the permission to use some of it.
The exhaust since it burns so well, and the way rocket mass heaters work, it will just travel along the floor under my living wall, into my storage/workshop area in the back, down one of the sides that has an opening for the back doors.
I will post a pic later of the Modified Kerosene Heater with the Propane Catalytic