I am going to check out that build here in a minute which may make my point mute
The tubing support in the camper itself is important, but so is the strength of the tube actually used. Imagine balancing a pencil on a cube, if you press on one side of the pencil it falls over. If you tie off one side of the pencil to the table that is under it, and press on the other side of the pencil you are now trying to lift the weight of the table. If the string doesn't break then the pencil surely will. The key seems to be figuring the weight (MAX) that will be on the cabover section, plus the weight of the cabover section. That will be the weight the tube would need to support, both in rigidity (so it will not bend), and sheer (so it will not sheer off at the transition point from main camper body to cab over). Then again I am probably over thinking this
I am liking the idea of aluminum framing though, its just finding what I need with out driving to Canada or something.