You might find a memory foam mattress is a better solution. We got an 8" one on sale we came across while away on a camping trip, 8 or so years ago. Shortened it with a bread knife in the CG. Very comfortable and deep-sleep inducing! Dog sure loves it!! :(
We tried a couple of 2" toppers on our mattress at home and they just didn't last and ended up getting a "real" foam topper mattress.
Only downside is that the thicker memory foam mattresses are a fair bit heavier. I had to relocate the two gas strut mounting locations on the under-bed storage lid so it would stay up. Easy to do tho.