We just got a new mattress for our 5er i can tell you that the width of a standard queen works fine. We found a local mattress store (Bedzzz Express) that can custom cut a mattress to length and so we had them cut it to 74 long (I would have loved to have the full length but our bedroom slide comes all the way to the cabinet in front of it so there is no room for a longer mattress). Oh and the new mattress is GREAT!
2008 31.5FBHS Jayco Super lite 5th Wheel
2003 F350 SRW 6.0 PSD