Both of my Jayco 5th wheels came with a standard 60x80 mattress. I bought a good Sealey a number of years ago and this is my 3rd camper that I've used it in. The Cougar TT that I had before the 2 Jaycos had a 60x76 queen bed and the longer mattress was not well supported at the foot end so I had to replace the hinged board under it with a longer one. That sounds like probably the best solution to your problem - you'll just have to remember to raise the foot of the bed to clear the dresser before sliding it in.
Chuck, Ann & the Boys (a pair of large labradoodles)
2013 F350 Lariat KR/6.7/CC/LB/FX4
2013 Jayco Premier 321 RLTS w/Atwood Levelegs (now removed)