gerrym51, thanks for shaking up my narrow focus.
You are right. There are several propane tanks that might fit in what was formerly a battery box under floor. I could look into those. I was concentrating on the standard 20 pound tank because they are by far the most popular size for backyard BBQers and seem to be everywhere for simple exchange at service stations instead of wait for a re-fill. Finding a competent place for a re-fill of a mounted tank under a B has always been a pain.