I would stick with Dometic, they serve a lot of the RV market and they have a wide selection. I would avoid anything sourced ‘offshore’ because you might want parts for it someday. As far as sizing goes, your insulation level will be as important as your trailers size. I’ve never had to size heating for a trailer but I bet a 15,000 btu unit would suffice, considering that a 30,000 takes care of a 24’ x 24’ garage quite nicely.
https://www.dometic.com/en-ca/ca/pro...imate/furnaces