After hitting just about every pothole between Arizona and Maine {Washington DC and Michigan being the worst by far} on an extended cross country trip 4 years ago we got a small leak during a heavy rain storm dripping down from near the top of the windshield. We were near Elkhart and stopped at the factory.
The tech found several of the screws the go up through the cab flange and into the bottom of the cabover had sheared off and quickly replaced them. When I got home I opened it up and replaced all of the screws with 1/4 - 20 ss machine screws {much larger} topped them with ss washers and nylon ss lock nuts.
When I finished torquing everything down the connection between the flange that is the top of the cab and the cabover is now rock solid and this of course recompressed the gasket. There is zero movement between cab and cabover and no chance of a leak.
It has been 4 years and everything is still nice and tight/solid. I have some pics somewhere on my computer at home {currently in San Diego settling my fathers estate - he passed last Sunday... sigh} and will be happy to e mail them to anyone who wants a detailed look at this repair{never had any luck posting pics here but if someone wants to share that trick I would be grateful}.