Have you looked into a small direct vent? I would install an Empire DV-210. I used to be a propane company salesman and we woould replace these with more efficent units regulary. Even if it needed parts the units are real basic "cheap". The one thing to look out for on a used unit is the heat exchanger. Most comon spot they burn through is near the viewing window. If they are run for periods of time without a tight gasket around the view window you create a lean fuel mixture. The little tourch area will burn a hole through the heat exchanger near the air leak.
Heater puts out 10,000 BTU
Empire DV-215 is 15,000 BTU
either one off Craigslist would work out in your shanty
Neither one requires electricity and can be run off a milliamp thermostat as opposed to the 1-5 dial built into the unit.