The key number should be right on the lock barrel, where you insert the key. Odds are the bay doors are a Trimark TM 500 key.
A 785B key was used on some other things, e.g. the water fill hatch. Maybe the engine bay too?
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL
Summers in Black Mountain, NC