I am not a huge fan of insulation. Unless you camp in below freezing weather often I would not do it. Adding a layer of foam and a cover of 1/8" luan adds some problems around the windows and doors. You will need to frame those out so the foam edge does not show.
Now with that said, if you can sew, I would get some of the bubble wrap that comes in rolls with the foil backing on one side. Get some upholstery fabric you like and sew it over the bubble wrap. You are making a blanket pad. This could be stapled or tacked up with upholstery nails. It's permanent but not really. My first teardrop was very well insulated. But the problem I found was when you roll against a wooden wall, even an insulated one, on a cold night, you WILL wake up. So a fabric barrier in my opinion would solve that problem without much space lost.
This is a cover I put over my cooler to insulate it. It's made just like I would do the walls. This does help keep the ice much longer so I know the insulation of the foil bubble wrap helps.
