I recently got an ARB Simpson III RTT, and experimented with substituting some older Thermarest camp rests for the OEM foam. Result was OK, but not any better than the original foam comfort-wise. The two separate air mattresses didn't fit across the width and part of the problem was rolling off or between them during the night. My secondary desire is to lighten the roof load as much as possible, as I was surprised at how much the OEM foam mattress weighs.
Has anyone tried the Cabelas house brand? I noticed they have a "couple" air/foam mattress which is 55" wide and 3.5" thick. Dimensionally it would seem to fit the RTT, be lighter, and if it sleeps better it could be great.