If you are not a believer in spray foam you should be. Particularly closed cell foam which has superior mechanical bonding, R6/inch insulation values, and outstanding sealing capabilities--it is waterproof.
I am restoring a 1967
vintage aluminum framed and skinned truck camper--it is a Barth.
It was stored for many years on two saw horses. At some point a branch fell on the roof creating a leak.
I marveled at the many of the Airstream restorations which, IMHO, turned out far better than the original designs. One can be creative with an old camper as it is easy to gut something worn out and no one would gut a brand new Airstream--that would be foolish. So I bought my camper with the idea of gutting it and starting over. Plus it is smaller and a bit easier than a big trailer
In my aluminum truck camper the floor was badly damaged in the back under the leak. I have some experience restoring yachts and while I might have been able to make it functional with repairs, gutting it and starting over is something I learned is often way easier than half measures.
Stripped to bare walls I found numerous mouse nests--more than a dozen--and I was glad I made the choice to gut it. I also found one live yellow jacket nest.
I was stunned to find the walls and ceilings were insulated with a thin coating of closed cell foam with no signs of leaks at the seams and windows. I give the credit to the foam.
Let me tell you it works. It sealed every place including the windows perfectly. All of the wiring worked perfect--this after 45 years. All the bulbs worked but one and most needed the contacts scraped with a knife blade. Most of the wire was bonded rigidly to the skin by the foam. In a few places the foam coated the wire and stiffened it up some. The wire didn't move or flex do it didn't suffer any mechanical stress. None of it failed. I credit the foam for that.
Aside from the hole in the roof, I found water damage immediately next to the cab over windows, so this damage was from windows being left open, or perhaps a small amount of water seeped in. There was water damage under the sink--probably the sink drain, and in one place on the floor where I found part of a panel popped loose.
Foam damage? It was pulled away in the middle of a panel in a few places like a bubble, but was still intact (no holes). In a few spots the loose "bubble" had torn and that might have been me while I was ripping out the interior. And there was a badly decomposed area under the sink where water probably pooled and the bees nested. The bee broke down the foam badly--I think they chewed it up. That was the only spot with real foam damage. In some most places the foam was extremely secure. I removed it on the wings and it was difficult--except for the bee eaten area.
The floor was filled with 1.5" of fiberglass batt type material--I assume to let any leaks escape through the bottom. I plan to insert rigid foam panels and leave gaps for water to escape if spilled.
I have a few pieces of sheet metal to replace, conduit to run, wiring to do, and then I will be filling the entire wall cavities with closed cell foam for approximately R9 insulation. It will require some trimming but I will gladly do that to maximize the insulation value of my walls.
I don't have dual pane windows, but I do have storm windows, and plan to build fitted rigid foam insulation panels over many of the windows. I want it to be easy to heat down to single digit weather. The small plume will help.
Another thing I discovered is the camper did have thermal conductivity issues at the screws. I found many of the screw heads rusted badly. I suspect they sweat and rusted from interior moisture in cold weather. To mitigate that I will be fastening many panels with double stick tape and avoid putting screws into metal-- instead where screws are needed for cabinets I will secure wood backing to the aluminum framing and then attaching cabinets to the wood, to provide a thermal break wherever feasible.
I removed the furnace and as much as I could inside to keep my camper light. I intend to use it for long distance travel. And I want everything new. I want it nicer than original.
Another thing I like about aluminum framing and wall skins is it will allow me to install a Sardine Wood Stove. I'll use fiberglass insulation behind it instead of foam. I will have a vented diesel or propane heater also. I really don't think I will need much heat if I build it right. Heat it with a candle? Maybe in warmer weather.
In any event, I wanted everyone to know that closed cell foam was being used 45 years ago and if you don't believe how well it holds up, I can show you. Mine is on so well I am debating whether to scrape it off and and may shooting the new over the old. Spray foam is one of mankind's greatest inventions.
My aluminum skin is thick. On the roof I plan to spray a layer of U-Pol spray in truck liner on the outside after sealing all the seams. With a 1-1/2" thick layer of closed cell foam followed by a 1/8" layer of marine plywood on top should make it one solid panel strong enough to hold up to large hail and falling branches or me waking around on top.
U-Pol (spray in bediner) is a similar product to closed cell foam, but stronger and denser. I honestly think that by masking around all vent and other openings on top, and spraying with U-Pol bed liner, I won't have to worry about leaks for decades. It can be tinted any color--white or silver to reject heat.
In my opinion, if closed cell foam filled all the wall and ceiling cavities, the camper plumbing was maintained, and the windows were well secured, this camper could last in prolonged harsh exposure for a very long time with minimal upkeep. I intend to find out.