First question is do you really need a mattress? We are on our 4th van and have had:-
- adapted sprung mattress with foam infill blocks to suit van profile over a plywood base.
- factory supplied fixed foam bed on slats.
- dinette made up as a permanent bed with 50mm foam mattress topper.
- presently,fixed sprung mattress on wooden slats with 50mm foam mattress topper.
Whilst are current set-up is the most comfortable I'd say there is not really a lot in it across the different solutions.

So a few things to consider. What van is it for and do you want a permanent set up. A sprung mattress would be out of the question due to weight and expensive being a special made to template fit or you'd have to cut deep foam to suit profile of the van. So realistically you would be looking at 10mm foam that you would need to put on a base of your making, either slats [IKEA] or plywood drilled with 50mm holes so the mattress can breath. Having used mattress toppers and foam IKEA mattresses I would make this my first port of call. They offer a wide range in cost and size that you can lie on to test. Removeable covers mean you can get the foam out and cut it to the correct profile with a serrated bread knife, or bandsaw if you have access to one. They will also be in stock for you to take away, nicely rolled up in a vac pack.
If you do go the foam route do not be tempted to go memory foam, they are very warm, something you will not like if you camp abroad in the summer.

Colin