A full sized queen mattress was one of the first mods we did to our Jayflight. I removed the existing bed platform and put 2x2's in its place, screwed to the floor. I used the plywood from the original platform and put it over the frame of 2x2's so that it was aligned with the new mattress at the foot and added some plywood filler pieces at the head. The plywood extends a few inches past the 2x2 at the foot but it works fine. That gave me 13" clearance to the bottom of the shelf at the head of the bed so that a new full size queen pillowtop mattress just fit. It's a HUGE improvement over the existing mattress...I don't know how anyone sleeps on those things. You lose the under-bed storage (well, not really but it's only 1 1/2 inches deep now) but it's well worth it, IMO. I'm 6' 5" so it really makes a big difference and was fairly easy and cheap (except for the mattress) to do.
2008 Ford F250 Super Duty, Lariat, 6.4l PSD, Super Cab. Long Bed
2006 Jayco Jay Flight 27.5 RLS Fifth Wheel