Here are a couple of intersting heater options:
1) Cozy Cabin Propane marine heater, 5000 BTU, requires 1" flue, 16"hx8"wx8"d, top is 4"x5" cook top and has auto oxygen depleation and CO shutoff $489.
https://tinylifesupp...-propane-heater
2) Dickerson marine solid fuel heater, 3000-8000 BTU, reqires 3" flue and fresh air vent, 14"hx8"wx10"d, burns 1" log or charcoal, $399
https://tinylifesupp...lid-fuel-heater