For those of you contemplating building your own shell,...you mignt want to read this.
I had a lap seam come loose!
The outer (.050") skins were originally attached to the sq tube structure with 3M VHB tape and installed according to recommended procedure. All of the sq tube was double or triple cleaned however,... something apparently didn't work as advertized (at least on the one lap).
Fortunately, I still had access to the inside of the structure when this happened.
Anyhow,... I decided to re-do the loose seam and "reinforce" all of the remaining seams/laps before installing insulation & continuing with the interior construction.
After doing some homework, I settled on a high strength urathane bonding material called Sikaflex 252.
This stuff is used by manufacturers of truck & trailer bodies according to their (Sikaflex) rep.
Anyhow,...I found a penumatic calk gun and used it to forced the sikaflex in & along side each sq tube member as I didn't want to attempt to separate the remaining (good)lap seams.
Also, as a test, I bonded a piece of scrap outer skin to a 8" long piece of sq tube & let it cure for a couple of days (& did the same with a strip of VHB).
I then tried to pull both apart without distorting the skin,...the VHB gave away, the Sikaflex didn't (My thumbs still hurt).
To get the .050 aluminum to break loose from the sq tube, I literally had to peel it off by curling it up with a pair of pliers.
Bottom line: if you're going to build a sq tube structure, I would highly recommend bonding the outer skins with this product.
I found it at http://austinhardware.com/
Go to "find" on the left side of the web page & type in "252". It's $8.52 per tube and for a 33' motorhome body you'll need aprox 24 tubes.

With both VHB & Sikaflex now holding the skins in place, I now sleep at night!