The walls were filled with 1" hard foam and held in place with expansion foam. If I were to do this again I would elect to use spray foam instead, the time savings alone would be worth it. The paneling is 1/4" plywood with a good face.
https://www.homedepot.com/p/100549260
CheapMessy and time consuming
See my youtube... Airgapped reflectix closest to van metal, then spray foamed foamular, cavities filled in with polyester fiber, floor has radiant barrier with air gap too, I made a video on it
Click vinyl flooring
Would NOT recommend. It was a pain and has definitely shifted/popped up in spots. If we did it again we’d use a single piece of sheet vinyl.