I have read many questions in the past about the best way to reinsulate an AS. I am currently reworking a 1976 31 Soverein. i have come up with what I think is a pretty good way to insulate for really cold weather. I have completely stripped the interior of the trailer, floor too. After cleaning and rustproofing the frame I used a spray in polyurethane foam insulation. (Tiger Foam.. Tiger Foam | Spray Foam Insulation Kits
) This offers about R6 for one inch thickness of foam. It is water proof and fire retardent and offers a bit of structural help holding everything together. I then put down R13pinkstuff and covered it all with Prodex Foil-Foam-Foil insulation that is a radient heat barrier. This foam foil insulation is about R14. For the floor I now have, on paper, about a R value of 33. I still have the forced air heat in the fresh water tank and the valve area of the waste tanks. I also used an electric tank heater for the fresh water.
In the walls I will be spraying the tiger foam at about 1 1/4 inches thick and covering with the foil-foam-foil, this will yield a little over R21 in a 2inch space.
You can see how the project is comming lifeinatincan.com