We are new owners of a 92 Newell and found the same issue with our OEM mattress. We found a replacement which was custom-made to our dimensions and is made out of the memory foam material. Ours is referred to as a "Cool-Max" memory foam mattress from MattressInsider.com which actually wicks the heat from your body rather than heating up like some of the original memory foams. Like the other members stated, we let it set in the unit with the windows open in our storage building for a week or so and also allowed it to fully expand before using. I think ours was around $500-$600 but has been all the difference in the world on that bed. Here's the link to the one we chose....
Luxury RV Mattress | RV Mattress Gel Foam
Hope it helps!