I think you misunderstood my post bro
Yep, I'll blame the wobbly pops.
What you want to do sounds good. There will definitely be some rotten framing to replace - the question is how far you want to take it. I did a few 70's RVs myself and the more I dug into them, the more bad wood I had to replace. Don't get discouraged by that though: it all goes into creating a solid foundation for the rest of your build.
With the foam stuck to the framing with some PL300 and a canvas 'sock' over the outside, I expect it'll be a pretty solid rig though probably not much lighter vs the aluminum. GPW's idea to use some aluminum sounds good, especially the 'abuse' areas like where it gets hit with road spray. There have been some issues with the canvas covering bubbling (also GPW, I think?) but, if memory serves, they settled down with no long term damage.
As for the marine sealant, 3M 5200 is definitely good stuff but I prefer Sikaflex 291 myself, especially for bedding things like windows. Since you're in BC, I can get it for you if you can't find it locally. That holds for anything marine, PM me your shopping list. Beware of buying it in a hardware store since it does have a shelf life.