That's a myth. The only modern coaches built like that (to my knowledge) are the Airstream and Avions
That alum. (or fiberglass, depending on year built), if you push in on it is backed with something, right? It's luan. It can, will, and does delaminate if not properly cared for.
The skin is bonded to luan for stiffness, and rivets go through both to hold them to the frame/superstructure. So granted, you won't see the huge bubbles you see on some coaches - but if you know what you're looking for, the trouble can still be there. Look for bulged material around the rivet heads indicating the luan has swelled or filled with water then froze. Or areas the skin can be pushed in slightly prior to contacting the backing?
Interior walls are the same way. The interior paneling (luan) is bonded to 1" foam board, which is bonded to the framework/superstructure. That can come undone, makes for a mess as well.
Same story with the foam backed vinyl headliner. That foam is starting to break down on some of the older coaches, causing the vinyl to sag - with no help from a leak required!
Bottom line, you still have to keep your eyes open on the HR's, even if you don't spot the trouble as often.... Don't assume!
1997 37' HR Endeavor, 275hp Cat, Freightliner
03 CR-V Blue Ox, Ready Brake