As John suggested go online to find a source for the foam. We have replaced the foam in multiple RVs over time and they cut to the exact size you want and will advise as to firmness. It does take a while to get, but in my estimation has always been well worth it.
Steve and Andra
2012 F350 6.2 gasser SRW LB
Fab Fours front and rear in case we run into a rhino
2019 Northstar Laredo SC