Any of your suggestions would work. You would have to make sure that the glue you use to adhere the foam to the walls will not melt the foam. Scamp
trailers uses Reflectix (looks like aluminized bubble wrap) insulation with a marine headliner material glued over it. I believe the R-value is somewhere around 6 or 7, but we never had any problems with this in our two 13í Scamps, and now in the sleeping area of our Lil Snoozy
. Most of the heat loss is from windows
and vents. Reflectix cut to friction fit over the windows
and vents at night really helps insulate.
Dave & Paula