So yes, in the end I went with a Dricore subfloor. It's 2' x 2' tongue and groove OSB with a hard plastic underside to act as a moisture barrier. Intended for concrete basements. Compared to the alternatives it was a pretty expensive route to go but it went down pretty easily and I have to hope that it will have benefits down the road. If not, who cares. I went a slightly different route than normal and I'm happy with that. I used 20 of those for my 11' x 6' space.
That's the first third done. I consider putting a 1/4" underlayment on top of that to be another third. And then the final third will be when everything else is laid out and I put down vinyl planks in areas still visible at that point.
I suppose I should mention that before I laid down the Dricore I removed all the original wall panels and old insulation. The old insulation was often piled up at the bottom and useless. I also removed the AC ducts that were in the walls and put sheet metal over the associated AC holes in the floor. The metal walls with no insulation or panels to cover them really radiate heat in the sun. It was a real lesson for me and now I'm thinking about beefing up my insulation plans.
My van is a 15 passenger so had real wall panels. It's amazing how much taking those off changed the van. It changed it from an old 15 passenger van to a blank canvas. And then to have the subflooring installed. Very inspiring and I can start picturing the rest.
I really wish I had some pictures to show of the work done so far but I don't. Instead I will leave you with some long overdue pictures of what the interior originally looked like when I bought it.