This is great stuff to use between your siding and the plywood if you're doing a rebuild. It will reflect heat back out and keep your Scotty cooler in the summer time and keep heat in, if you're heating your Scotty. You can buy it at lumber stores, but much cheaper to buy it in bulk from here.
Nancy is now recommending filling in the spaces between framing on the front/roof/rear with a foil-one-side insul board. You can get it in 3/4" thick and it fits perfectly for Scottys that have the 3/4" framing on top of the inner skin.