Propane furnance, same as a motorhome or 5th wheel etc use. Atwood is an awesome brand.
Do a search on here, theres plenty of threads discussing heater's, furnaces etc. Furnace is the only way to go !!! If your not in a hurry, i'll sell you my 2(3) place in 6 weeks when my new trailer gets here. All aluminum frame, and over 6g's worth the goodies. 11k