I know this doesn't address all your concerns, but some RV suppliers use oddballs. King is 76" x 80", Calif. King 72" x 84". I have read about other "King" (European) sizes. You could state your case to the seller and tell them you want a standard King or Calif. King. Use other RV's as an example.
I've never been comfortable with the RV supplied mattresses...they are the first things to go...went to a Numbers air bed.
Suspenders will work.
I see from the Trackrig link there are RV Kings 72" x 80"...wish they would state the seam length.