We bought a 4" memory foam mattress topper from Overstock.com 10+ years ago for the beds in our trailer. We liked it so much we bought one for our bed at home.
When we bought our current 5th wheel, we transferred the old topper to the new bed.
Check out Overstock.com. They still carry 4" memory foam mattress toppers.
'13 Excel Winslow 34IKE
'12 GMC Sierra 3500HD
Previous RV's: '03 MobileScout Titan 29DBS; '01 Sunnybrook Lite 2708 SLE; '99 Coleman Utah
Previous TV's: '02 Chevy Suburban 2500 8.1 4.10; '99 Chevy Express 1500 5.7