the mattressunderground.com
sleeplikethedead.com
Two great sites on the topic of sleep, tens of thousands of pages of information, and a forum to ask any mattress question you could possibly dream of.
In our case, we bought a older motorhome a few years ago, and it came with a Tempur-Pedic queen. This thing was nine years old at the time, and showed zero evidence of use. It is an old school style, about 7" thick with three inches of high density, high weight foam, bonded to another 4" of a lighter foam. After a few nights of giving the thing a test run, it became evident that it was the best sleep we ever had. We drug the thing into the house and have been using it ever since. It appears that it will last for many years to come.
Sadly, tempur-pedic products are grossly overpriced, and now a simple mattress like ours is no longer offered in their product line. There are tons of foam mattress options out there, and it is possible to have one built for a reasonable price, to your exact requirements, by smaller manufacturers that don't rape you, in order to spend silly amounts on marketing, or dealer profit.