Here is another place to look for a mattress for a van or a truck. We have used a memory foam type mattress for the past 4 years and we are still very happy with it. This company will also custom make a mattress to fit your area.
I've always used a innerspring mattress that I've bought from truck-n-tow, BUT at last years expo I tried the sleep dog and that will be in my next van for sure.
What we found running team is that we had a problem bouncing on the bed with just springs. We found ourselves trying to sleep while bouncing up in the air and coming back down. With the memory foam we no longer bounce while going down the road while sleeping.
I use an inflatable thermarest pad. I add a second thermarest foam pad if its too cold. I use both either on a camp cot or on the floor. I like this set up, because they are light and disappear if I'm running short on space.
I had an all-foam one made by the Original Mattress Factory (the local company Terry recommended) .... 36" x 66" .... it's pretty decent, but I should have ponied up and got it another inch thicker than what I did.