I believe these are generally appliance-specific rather than trailer-manufacturer specific. For example, the 3-way refrigerator
vent is a Dometic part (I think I've seen these new at either Vintage Trailer Supply or Scamp).
Probably not all of them will be available new (for example they don't make the typical Trill furnace
anymore), but I think if you are going to find them you will have the best luck at RV places going by appliance type and brand.
Other thoughts: You could also try RV salvage places. Or perhaps try to salvage yours with paint
. Or take the remains to a fabrication place (or do your own as mentioned).