Adam,
In my 13 ft
Burro I replace the factory vented heater with a
Olympian wave 3 unvented heater and it was more than adequate for the cabin size. You may want to go up to a Wave 6 if you plan on much sub freezing campiing. Shop around, I see the prices of these all over the map.
Trim Lock is easily available at any auto upholstery shop. Check around your area.