Unless the bed in your camper is in a box, with a wall directly at the head and foot (like in a corner), you can add a full Queen mattress. Just let the extra length hang over the end. You loose a few inches at the foot of the bed, but getting a regular Queen is much easier than getting a short queen.
About the selection of a mattress? Why not get one that is identical to the one in your house? There's nothing special about RV's that has to be different than your house.
In our last camper, we replaced the short queen with a long queen. It was a Serta, identical to the one we had in the hosue. We kept the original mattress and when we traded for our current camper, we swapped mattress back, thinking we'd put the new one for the old camper in the new camper. Found out we didn't need to. Our Outback came with a really decent mattress! It's extremely comfortable. We were surprised.
What did we do with the one we bought? Well, we tossed one out on one of our guest beds in the house and put it on there. Now both our Master Bed room and guest room have the same mattress, except one is a King and the other is Queen.
So you know, what ever you are using in your house, just get the same for the camper. Problem solved!