Heater or furnace?
I would go with a Suburban NT-30SP or NT-34SP, Iíve heard people on this forum say 1000 BTUís per foot of trailer.
I did a 1975 Tradewind last year and could not find a direct fit replacement furnace, I ended up going with a Suburban NT-24sp. This is how I did it.
I would also recommend buying from Palomino parts:
800-297-5830, there ultra inexpensive, and will ship.
Parts needed: Fan for roof vent or whole vent fan assembly, vented catalytic heater.
My blog for the 1975 Tradewind restoration
, Has not been updated in a while but I am still working on it.