Plenty of discussion on the forum about this issue. LOTS of solutions.
Personally, we added a 4" memory foam topper and it has worked as hbravo stated; a "stopgap". A pretty good one at that, but the OEM mattress underneath (Denver "pillow" top) is getting to the point where we're going to need a new one. If I were to decide right now today, I would get an 8" foam mattress from Amazon in "Short Queen" size and retain my topper.
While the Sleep Number is VERY highly recommended, we just don't spend enough time in the RV (unfortunately) to justify the purchase. If we spent more time, it would be right at the tippy top of my list of replacement options.
2014 Jay Flight 28 BHBE
2015 RAM 2500 6.4L HEMI, Tradesman 4x4, 3.73
Blue Ox SwayPro (BXW 1503)
Upgraded from an REI internal frame backpack and a Eureka 1/2 dome tent!