The original dinette seat cushions in my 26BH were really uncomfortable. The cushion foam was so soft I'd "bottom out", literally! So I fixed the problem on one of the seats.
I learned that Jo Ann Fabrics sells firm seat cushion foam, and they will cut it to fit, charging for only what you get. I got enough for one cushion just to try it out, and I'm happy with the results.
And there's a 40% off coupon on the Jo Ann website ( http://www.joann.com/coupon
), but it's only good through tomorrow.
I will probably go into town today and get enough for another cushion. It's pricey stuff for the 4 inch thick foam - $60.99 per yard, but in my opinion, the comfort it provides is well worth the cost.