In the tiny space you're talking about, a woodstove would likely take up too much space and be a danger, as well.
Had an 1958 18' Airstream I used in British Columbia and the western US; it was equipped w/ a built-in propane convection furnace which was fed outside air, for combustion. No danger to occupants. Nearly effective enough. Fairly large. My 1967 24' Silver Streak has a similar system. My boat has a diesel convection cabin heater which is very efficient but, again, too large for your app.
One of the small, portable catalytic heaters seems more suitable for your tiny trailer.
