RV heating and cooling using multiple sources of fuel
Don't make perfect the enemy of the good, or the project never finishes.
Space dimensions do not decide HVAC requirements. Insulation value x surface area does x air leakage.
So it's all about the insulation.  Walls with 6&quot; of aerogel and good door/window sealing will be a completely different energy picture than J random converted step van with #16 aluminum on 3/8&quot; plywood and a 1/4&quot; crack under the door.
Where you lost me, however, was &quot;multiple fuel firebox&quot;.  Firebox????   It sounds like  you want to rocket off into space and design your own stove from scratch that will take everyfuel.  Forget it, that's a Manhattan Project to do with any level of safety or sophistication.  I assume you want a heater that you can refuel without getting your pajamas wet, yet that will also not kill you from carbon monoxide?  Generally you have 2 options:

Build a hillbilly nightmare that will burn the place down or gas you
Buy a quality, efficient product off the shelf that Just Works and you can run it while you sleep so you don't have to sleep in long johns and spend the first hour of the morning trying to restore heat while your hands go numb.

Since I gather you don't want three totally separate furnaces in that compact space, you better select one that's a safety and functionality winner and stick with it.  There's not going to be any shortage of propane or diesel anytime soon.
I gather you'll probably want to cook.  Think about that as part of the heat source decision, pretty dumb to need 2 different fuels for the two tasks.
When you're RVing, you don't need air conditioning.  If you really want it, cover the roof with solar panels and you'll have plenty of power to run a compressor based A/C system.  You'll want a sensibly sized battery pack anyway, because if you're too fancy to live without A/C I'm sure you're too fancy to live without electric conveniences.  Running an off-grid electrical system, sizing panels, batteries, loads etc. is a topic all its own.
Solar plays well with A/C because the only reason you need air conditioning in the first place is solar gain.  Thus the need and the power supply arrive in same proportion.
I seriously doubt an ammonia based absorption refrigerator would even be capable of scaling up to air conditioning size.