I wouldn't bother with a topper unless your budget is really restricted. If a standard size mattress (whether full or queen) will fit on your bed frame, than just buy a new mattress. We ended up doing that in our previous trailer, and with our new Winnebago, we just donated the stock mattress to the Salvation Army and purchased a new mattress at Sam's Club, where you can get a really nice mattress for around $500-$600. Because the bed frame in our Winnebago butts right up to the entertainment center wall, we replaced the short queen with a full size mattress. If you have the room for a full size queen (longer than a short queen), it just depends on whether your bed platform will support it, or whether you can do a modification to support a queen mattress.
Otherwise, many mattress stores can order a short queen mattress for you. It seems that it easier to do than it used to be, and prices have gone down.
2012 Newmar Bay Star 3302
Blue Ox Avail
BrakeBuddy Advantage
2015 Malibu
"Get busy living, or get busy dying."
Andy Dufresne, The Shawshank Redemption