Hi boatnut! The mattress that came with the new 2306 was about 5" thick. Plus you can plug it in for a heated mattress! We would have kept in gladly, but because of my disability I needed a bit more. About 2 years ago we ordered an 8" short queen mattress. There are lots of places who sell them including on Amazon. Camping World, Camperpedic, and lots of others. Just put in custom RV short queen mattress. They will vary as to what they are made of so look for what you think will do you best. Many folks are using the memory foams ones, but my husband doesn't like them because they hold too much heat for him. Also, you might have to think about the Murphy Bed. Our mattress is a bit heavier than the one that came with the 2306. So the cable you have to pull to put up the Murphy Bed is even harder to pull than before. And it was difficult before, now worse. Prices will also vary. If you are thinking about getting a 2306 you might want to try out the one that comes with it. It is way better than the 2 or 3 inch rv mattresses we had in other rev's. Then go from there. Hope this helps!
Chuck and Bobbie
Our "Fur Kid" Roxie the smiling doxie is so amazing !