Hey! I was gonna ask that same question!!!
If I sleep on one side or the other, both my arms fall asleep
We considered, for a brief moment, replacing the standard foam matress with a standard one, but of course the clearance between the bed and the roof lights is almost nil.
Our idea, and we haven't tried this yet, is to buy some of those "ThermaRest" pads -- the inflatable ones I always drooled over when backpacking. They make some pretty thick cushions now. They roll up and go into a stuff sack when unused, but basically act as air cushions (up to 2.5 inches thick).
Has anyone given these a try?