Have you thought about the self-inflating foam+air therm-a-rest type backpacking mattresses? These can often be very comfortable with the added advantage of very small storage size. The valve can be used easily to adjust the cushioning level. I have a small 14x14 size I use as a chair pad in long drives... But they have extra wide versions as well. They're made by a number of different manufacturers now, but these guys were the original I believe...
Tow of these side by side and you'll have the camping version of a sleep number bed! You could probably try them both above and below the dinette cushions to see which worked better, I'm guessing above is probably the best though.
The other option would be to replace the foam in the dinette cushions themselves with a different foam makeup... Perhaps a firm foam not quite the full thickness and then add some REAL memory foam to fill the rest of the way. Keep in mind there are a lot of products called memory foam but most are not it, if it's inexpensive, it's not memory foam... If it's not made in the USA, it's also probably not memory foam.... But anyway, you can make the foam composition you want to make sleeping better and just zip that into the dinette cushions and that would get you something better than the very stiff and unforgiving foam that is in those cushions now.