While a rocket mass heater would be a very efficient way to burn wood, getting the mass to radiate the heat into the camper might pose a problem.
Small, old campers are notoriously drafty, the most bang for the buck is usually by spending your time insulating as well as you can before spending any money on adding more heat. Adding insulation is a one time thing, instead of gathering wood all the time (and eliminates concerns with a fire in a small space).
Everything you can do to seal up drafts and add insulation pays off quickly. A little caulking and duct tape usually go a long way on campers. Stuffing newspaper into cracks and adding insulation to the roof, walls, and floor would help if feasable. Heavy curtains over the windows at night helps a lot. (1" blue foam makes excellent removable panels to cover windows at night)
a couple of sheets of foam on the roof covered with a tarp or polyethelene then weighted down with something heavy is a pretty common fix for heat escaping through the thin roofs on campers (but a bit unsightly if that is a concern) along with some skirting or straw bales to keep the wind out from under the floor.
Many times you can get the materials to insulate very cheaply or for free by scrounging them.