Your comment on insulation is right on target!
Yes, I think it may be a good idea to seal the outside edges and paint the exposed wood with a sealant. Leaving a small gap for the steel to breathe is also a key procedure to avoid moisture building up on the frame and subsequent rust.
About your comment on Airstream engineers, I'd like to make an observation. In 1975, I invited Bill Hewlett of Hewlett-Packard to speak at a business club at the university, and he came and was very "entertaining" to us as senior business majors. At lunch, he told me that H-P hand-held scientific calculators were built with obsolescence in the design. This was purposeful, he said, because no company could continue in business without providing customer support and repair service for existing products along with building new and better products. I think the survival of H-P is a clear indication of Bill's business mind.
So, Airstream probably thinks in a similar vain. If Airstream built trailers or motor homes to last "a lifetime", then a good part of its profits from new sales and repair would not happen; thus its survival as a going concern could be questionable.
Thanks for your thoughts.