Our coach came from the factory with a king, but we wanted a queen for more walking around room and some space around the small “bed stands”. BCS simply replaced the mattress, but (don’t quote me on this…memory dims) may have trimmed the edges down on the plywood sheet it sits on. Whether a couple inches of plywood on either side is particularly necessary for king support, perhaps; but a new perfect-fit sheet of plywood might be all you need if all coaches come prepared for a king mattress.
That’s my assumption, maybe misguided, since our coach is a shorter-than-most Monterey, and yours, Fred, is a much larger floor plan that I’d assume came originally with a king.
Joel