Almost the best part!!
Called up Fantastic Vent and said, "gee, I have an Airstream and I hear you have inside trim that fits the ceiling." I happened to mention that I had installed 5 of them. Their response was, "what's your address, they'll be in the mail today." That was Thursday. They are here. The sound you hear is jaw hitting floor.
Karma Karma Karma!!!!
It's the full trim piece, not just a little gap filling insert. More karma! Oh, by the way, could only get white units at Campers World, but the great people at Fantastic sent beige. YES! OK, so I'm giddy, so sue me.