We don't find short beds acceptable. Our previous RV had a 74" long "queen" mattress which extended about 4 inches past the end of the plywood. I extended the plywood platform to 78" which we had room for. Then we added a 5" thick piece of dense foam cut to the height and width of the mattress. This was placed at the head of the mattress which gave a 79" long length, just short of a regular queen to make installing fitted sheets easy. With a good mattress pad covering all, the transition from mattress to foam was not noticeable under a pillow.
If you have a slide out that leaves no room for a 79 or 80" long mattress, you could lay a sheet of plywood on the lower half of the bed platform that you just slide out to support a longer mattress. Then, using the foam extension I described above, just fold the foam up from the head, and push the mattress and the plywood extension back to the shorter length for retracting the slide-out. With a fitted pad and sheet, you wouldn't disturb your bedding much, and then when you set up for sleeping, all you have to do is pull the plywood extension and mattress out and flop the foam back into place at the head. I will do anything to have the full queen length because of my height.
'09 Pleasure Way Lexor, Chevrolet Express 3500 chassis, 6.0L V8
Last RV, 2005 NRV Dolphin 5342 (2007-2014)