After 11 years, the dinette cushions in our 2005 19' Bambi were getting too soft; and when one sat on them, you sank through to the wooden frame.
Upon removing the foam cushions from the upholstery covers, the cells in the foam were breaking down; so we replaced the cushions with foam we found online.
Vendor's description for the foam we used:
Lux Foam-HQ (High Quality) - Firm foam for regular or heavy use. Lux Foam-HQ is a high quality foam suggested for seats, chairs, cushions, benches and mattresses. It has a typical life of 16 years. (2.8LB /ft3 density with a 50LB ILD)
The new foam is firmer than the old worn-out cushions, and (to us) it is much more comfortable.