SecondLife,
I really like your project. Building my own travel trailer is one of my fantasies, and will remain a fantasy, unless I miraculously began to catch up on projects already started.
I am glad you caught the overweight issue and are determined to conquer it. I always think of all my projects as proto-types and each needn't be perfect. That thought gives me the courage to move on and paint a fresh new canvas without regrets.
One other issue most often missed, even by RV industrial design engineers, is that of condensation damage, especially when outside weather is significantly cooler than inside cabin temperatures. To master this, you will need to provide a very low permeability vapor barrier on the warm side of the insulation where the vapor is still dry. I know it is common practice to assume that closed cell spray foam is adequate, but the truth is even closed cell foams slowly absorb humidity. And it is very difficult to get it back out of the tiny porous cavities, causing accumulation and some corrosion (or rot in the case of organic materials such as wood).
It is recommended to install Polyethylene sheeting (commonly sold in home supply stores) just under gypsum wallboard in full size homes. Or in this case, under interior paneling in campers. It might initially seem like an unnecessary step, but consider for a moment where a major source of humidity in homes come from. It comes from occupants.
After the 1974 oil embargo, science minded leaders in the USA realized that we needed to have much more efficient heating in our homes. The solution was to seal the homes much tighter, including doors and windows, to retain heat. The first Federal government specs (those dreaded regulations again) for raising the fitment tolerance of these manufactured items was mandated to reach this worthy goal. It worked sort of.
Unfortunately, as soon as the warm air quit leaking out of homes, the stale moisture was trapped inside with it. Window companies began to have warranty nightmares when window condensation quickly ruined the brand new wood window frames. Consequently, the manufacturers did a study on humidity, and discovered that each person accounts for the release of up to about one gallon of moist vapor per day. A family of four released up to four gallons a day from breathing, cooking and bathing. Even though our RVs are much smaller, we creatures still release similar amounts of water vapor that must somehow escape our tiny homes. Imagine dumping a bucket of water on the floor everyday and hoping it will dry out before the next.
There is this terrible tendency for vapor to be attracted to the cool outer walls, and if they are even slightly porous, the dry molecules of water vapor began to quickly migrate through the wall where they soon reach a cooler core. There they condense (become wet) and cause trouble.
RV walls have been known to warp and peel from this insidious process. One might note that window panes do not allow much migration, and there the water condenses on the surface where it can be observed. But the same thing is occurring inside the walls where it's effects cannot be easily seen. Until it is too late.
So I will say now, I believe that early planning for humidity control is an important part of the overall engineering blueprint.
Good luck in your endeavor. I look forward to sharing your progress.
Wes
