As for wall construction, different manufacturers have tended to use different techniques for a long time; there's no one general time, so far as I know, when materials changed. (The same goes for the structural framing, as well--whether it's wood, or aluminum, or steel.) I personally wouldn't run away from a luan substrate if the RV is in sound condition.
The joint between the bunk board and the cab proper doesn't seem to generally be a major source of leaks. I believe typically these are assembled with a rubber gasket between the two, and screws holding the bunk board to the chassis cab roof. Presumably they tend not to leak partly because there's not much way for water to work its way in (since it's protected by the overhang) and partly because the weight of the cabover structure keeps good pressure on the seal. Much more commonly troublesome are the various seams at the roof joints, the running lights, and the front window in the cabover if there is one.
