I know there have been previous threads relating to leaks at the drip rail, but I thought it was worth adding my recent experience.
Previous owners of my MH had obviously fought a leak over one of the bedroom windows; until today I had handled the leak by dealing with just the caulk on the rail. Today I removed the black plastic molding that covers the screws holding the rail in place and figured I would tighten them before re-caulking. To my surprise (and concern), neither the original screws and nor those added by previous owners could be tightened without breaking them off. Apparently no one during manufacture or subsequent repair had thought to use stainless screws and rust had gotten to the steel ones that had been used.
I removed some of the existing screws, those that were strong enough to be removed in one piece, and sank stainless ones in their place. In other areas I drilled through the drip rail and installed new stainless screws. When I caulked I made sure to cover the screw heads and holes where screws had broken even though they are all supposedly protected by the black plastic molding.
The good news is that I was able to very clearly tighten the drip rail much tighter than it had been. That gives me confidence that I have a shot at fixing the leak. The "bad news" is that I realize I need to go around the entire coach and install new stainless screws. I don't plan to try to remove any of the original ones, I will just drill holes for new ones and tighten them down. #8-3/4" will probably work best; there isn't enough clearance to reliably run in 1" screws. I'll use whichever works best at each location.
As for caulk I used clear "Life Calk" which I purchased at West Marine. It is a combination urethane/silicone caulk designed for fiberglass which I've had very good luck with previously. The clear version is difficult to find and is only sold in small tubes. For the rest of the drip rail around the coach I don't intend to recaulk unless I see signs of damage.
