I put a 4 inch topper on the Denver " mattress " in our 222 Octane SL. The factory mattress, has about an inch of padding on top of the plywood, just awful! But with a 4 inch topper on top of a 1.5 inch topper, on top of the Denver junk, it's pretty doggone good!
2016 Jayco Octane 222 SL
2002 Ford F-250 7.3 PSD