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://tinylifesupply.com/propane/cozy-cabin-propane-heater
2) Dickerson marine solid fuel...