My '98 Coachmen does have a couple of aluminum framework bars across the front of the cabover that I assume are welded into the rest of the framework. They go immediately above and below the front cabover window. I would think that any cabover with a front window would need some sort of framework around that window, be it wood or metal, and not just the foam and fiberglass and luan or whatever the walls are constructed of. There's also some wood formers for the lower curve of the front of the cabover. Manufacturers do, of course, differ in their handling of such details.
I've never noticed what I would consider excessive flexing. There are occasional creaks from somewhere in the general area (which may just be the gasket between the cab roof and the house part).
And like the others have said, the vinyl trim in the corner molding is there mostly for visual effect; it's not waterproof, and isn't the primary seal for the corner. The real sealing comes from butyl (or something similar) underneath the metal corner extrusion.
