Tyvek Thermawrap plus spray adhesive to mount the Thermawrap to the metal walls of the Sprinter, white Tyvek scrim against the metal, and the white fluffy material facing inside the Sprinter.
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