I've suffered with back problems for several decades.
The bed in our old Ace, which we had for 18 years, was wonderful. The foam was just 4" deep so you could feel the solid base underneath. My back was so much better after a week in the caravan.
The bed in our current Pageant is 6" deep, sprung upholstery, which is much nicer to sit on but far too soft and unsupportive to sleep on. When it's really bad I sleep on the floor!
I'm seriously considering "throwing away" our present cushions and having some firm, thin, foam cushions made up!