foambymail.com is great. I've had a bad back since I was a kid loading and unloading semi trucks in Columbus, Ohio, but I haven't woken up with a back ache since I designed my own mattress through them. Plus, I used the same "formula" used in most high end mattresses, but paid less than $500 for it. If you have enough room, I,d suggest a 3-5" standard foam, two "eggshell" layers (they're thin, but they're only there to let the mattress breath), topped with a couple different densities of memory foam. Keep it to under 12" and it fits nicely in one of their mattress covers and doesn't really need any kind of base or box spring under it.
I haven't used it camping, but I would say that memory foam mattresses hold heat so they keep you warmer during the winter, but could be less comfortable during the summer ... although the eggshell layers help release the heat.