The Vanagon's metal currugated floor is the worst possible kind of surface to build on. Obviously you can't just throw a pad on it and expect it to be comfortable. I tried and it didn't work.

One of the major problems with this floor is keeping the cotton cloth tight through varying levels of humidity. If you hang out a lot on it like I do, it will collect a lot of salt and attract moisture, causing looseness. To solve this problem, use "Bungee Cord", available at sports, camping, hunting or surplus stores. Here you see 1/4 inch diameter cord but that is too large. Next time I will use smaller diameter.
Get "picture hanger" hooks and #6 sheet metal screws from a hardware store. Bend the hooks so the hook part sticks out. Drill and screw them in about a half inch above the floor. You should be able to slip the cord under the hook.

To level out the uneven surface, scavenge pieces of Coroplast (from old signs, for example) and made strips. I cut them crossways as I assumed that would be less compressible. Fill in the troughs with two layers, that will bring the floor up to level.
You will need one large sheet of Coroplast purchased from a plastic supply company. Also get a can of appropriate glue.
Cut the large sheet to fit just right and glue all pieces together as you lay them down. Let the adhesive dry overnight with some weight to hold them together.

As you work on the Coroplast, be careful as it is easily collapsed by knee pressure. After you put the foam on the pressure will not be a problem.
This subfloor you see here is actually 6 years old, it has been an incredibly good performer. The black spots are a remnant of a failed experiment, I had used 1 inch thick Rubatex foam however after 2 years it collapsed in the summer heat with me sitting on it. Obviously Rubatex is not the right material for durable resiliency.

Here is the floor with a sheet of 1/2 Poron® foam, modulus 30, density 15. Poron is an extremely tough and heat resistant urethane open-cell foam. I purchased this sheet from Stockwell Elastomerics and it was $180. Divers will know Poron as the rubber used to make diving suits.
I did test a piece of this material for 6 months before purchasing it so I know it will perform well under high heat and mechanical compression. I also tested the THK HT-800 material but it crumbled under mechanical agitation.

This is plain cotton denim purchased from a fabric store. I have use 1/4 inch grommets and the elastic cord keeps the cloth under constant tension regardless of how wet it is.
The eyelets take some technique to install properly. Use an awl to make a hole from the bottom of the cloth, do not use a punch to cut a hole. Use a tapered tool like a needlenose plier to expand the hole from the underside until the large half of the grommet will fit in from the bottom. Then put the small half in the top and crimp it.

Here is the completed floor with the denim stetched tight. The part where the door is I haven't figured out how to fasten it down there. When I do I will post it here. My previous floor used velcro but it was not a robust solution.