Rather than deciding on a final floor for the van, I decided on some cheap 1⁄4” Maple plywood from Lowes and remaindered paint. That will let me build a template floor and test it out before deciding on a final floor.
After reading many forum posts on insulation, I’ve settled on a hybrid approach. I’ve got 50 ft of SM600L Thinsulate for the walls and ceiling, and am using polyiso for the floor.