Let me share some experience about luan and insulated foam boards. Both of these products will give off vapors (especially during the summer). Make sure when buying foam board products that there will not be any harmfull styrene or other gases. If you use luan, you will want to paint it to reduce fumes from the glue. You can also get the white hardboard, about 1/8th of an inch, that is used on bathroom walls. It is tough, relatively easy to work with and will brighten up the interior. I believe you can purchase it at Home Depot. If you do not want to spray foam in the van, then you may want to go with the "bubble wrap" and then cover with board. You do not want to use fiberglass batten insulation, since it will hold water, resulting in lost insulation value as well as mold. Also, you will want to get rid of that felt backed vinyl liner on the floor of the van. It is another place for water to accumulate, plus it will smell bad and be a breeding ground for mold.