The top frame dries out fast even when roof leak.
Styrofoam keeps it dry as well. I had badly repair cap on my 10 years old camper when I bought it and pulling it out discovered that some of semi-structural blocking was completely gone. They put fiberglass in the cap, what kept it wet year around.
I am nicely surprised that on your picture top stack looks like 16" or more of structural lumber?
But then with the windows look like your cabover has no structural lumber at lower line?
I hope you are familiar with wood laminating.
It not only makes easy repairs, but make whole thing much stronger without adding much weight. Waterproof as well.
