Talk to these guys:
The Foam Factory, Mattresses, Memory Foam Toppers
I have made three beds from their stuff. It's basically a knockoff of Tempurpedic at a fraction of the cost. I have made a custom bunkbed for my kids in my 34 footer using their products (4lb foam for the upper mattress, 4lb foam for the lower back rest, and 5lb foam for the lower mattress which serves double duty as a couch during the day), as well as a 5lb topper on my King bed in my house.
I've attached a pic (not the greatest) of my custom front bunkbed that I made to replace the gaucho across the front of my 34 ft triple. It works great. Kids love it, and I've slept in/on it a few times.
It's great stuff, and if you want to save some money, just order a rectangle in the next bigger standard size than what you need, and cut it yourself. I just used my J.A Heinkles 12" chef knife and it sliced it like butter. It's very easy to make any shape you want.
Best of luck,