there are three basic wall-mount choices (ignoring the portable heaters like Mr Heater Buddy and Coleman's Black Cat series), vent-free, catalytic and vented.
Vent-free burns propane right there in the air, with safety features. Creates moisture in air and requires some ventilation.
Catalytic also burns propane right there in the air, with safety features. Creates moisture in air and requires some ventilation.
Vented burns propane inside the furnace
, with outside air piped in and exhaust piped out (standard RV furnace
is a variation on this). Does not put moisture in air and doesn't actually require other ventilation to run. May or may not have optional elec blower to increase efficiency.
Empire makes vent-free, Olympic makes cats, and Dickenson (West Marine catalog) makes vented.
Dickenson also makes a solid fuel vented furnace that will burn pellets, charcoal and small pieces of wood.
Be prepared for sticker shock on all of the above.
I have a non-RV intended Empire unvented, piped into my propane line. Sure is convenient just to turn a knob and press a button to start heat.
Least expensive and most flexible way to go IMHO would be a Coleman PowerCat, together with a Coleman Black Cat (prolly less than 125 USD total). Use both together to bring temps up and then just run the PowerCat. Stash em in the storage bin or truck in summer. Take 'em to the outhouse or shower or out under the awning
or whatever. Acquire fittings to refill propane canister from your big tank if you want to save more money.
Pete and Rats, currently warmed by electric (quartz rod) heater plugged into friend's house