We used a 1" topper on top and 1/2" exercise pads underneath our front bed in the 21SS. It was a struggle sometimes to get the door closed - but it did work.
However, we eventually gave that up as I'm just too hefty - I could feel the door when I was on my side. . . So, we went to an air mattress and it became a non-issue.
When we got the 23IKSS we put the mattress in storage and put in our air mattress setup. (We had to go to 1" rigid insulation to even things out between the door and the front cabinets - only needed 1/2" in the 21SS.)
2016 Roo 23IKSS / 2011 Tahoe
2011 Roo 21SS / 2011 Tahoe
2006 Bayside / 2006 4Runner
1999 SantaFe / 2001 Sienna
1967 Nimrod / 1998 Outback