6" thick seems awful thick! My foam is standard (non firm), but it is 3" thick. I got it from a "Mattress Foam & Upholstery Warehouse" place, or something like that. They do all kinds, custom cut in front of you while you wait, and they do memory foam. If you could find a place like this near you I would recommend trying some different things out. Just for comparison though, 6" is about as thick as a decent residential mattress, which would take up a ton of space in a van.
Good luck, I'll be watching this with interest.