We have the 23MBH with the Murphy bed, and needed a new mattress too. We went with the Classic Brands 8-Inch Cool Gel Memory Foam RV Mattress, Short Queen that can be purchased on Amazon for $231.00 with free delivery. It comes in the Short Queen and is 8 inches thick. It is very supportive. I would think you could easily do up to a 10 inch thick mattress. The 8 inches had plenty of extra room (thickness). We loved this mattress and slept on in over 10 nights now on several trips.
2014 Ram 2500 MegaCab Turbo-Diesel
2015 Jay Flight 23MBH
Former pilot to the rich and famous. Current domestic engineer, head chef, head laundry officer, head dog walker, and pilot volunteer. Lover of tacos and soccer