REI camp pads in the 3.5" size can work well, and are portable. Add a 1" memory foam topper for extra luxury.
Get a 4-5" foam mattress from Walmart, Amazon etc. Cut it to fit using a electric carving knife. Then cut into 2-3 sections. Have covers made for them. Sunbrella or other outdoor fabric works great and is very durable. Another option would be marine vinyl. It is nearly bullet proof, waterproof, and patchable.
The sections can then be folded up or stacked out of the way if needed. They could be used for lounging outdoors even.
Edit: for hanging stuff in the van, a rope run through a piece of PVC pipe can be closed in the door (with the pipe outside). This will likely support a small hammock with ease. Rubber or vinyl tubing around the rope will prevent damage to the paint/seals.