Roof channel molding

[Roller4tan]

I'm in the process of recaulking above the roof channel along the length of our bighorn. The caulking is checked and powdery when scraped. No gaps that I can tell and the butyl caulking under the channel seems good. On the off door side at the rear corner,I noticed some rusty discoloration along the bottom of the moulding. I stripped the caulk from the screw holding the end of the channel moulding and as I removed the screw water came out from under the moulding. I pulled the moulding out about 6 feet to where the screw heads were clean and white. I then removed the rusty headed screws. The threads were still shiny, so I knew water hadn't got passed the heads. I added caulk around the threads and screwed them back in tight, all held with no stripping.
I'm trying to figure out how the water got under the moulding. Like I said there appears to be no gaps in the caulk above the channel. But if there was it would go under the channel, not in it.

 

[caissiel]

Mine rotted the rear wall because the water penetrated through the roof side trim at the back. Screws were rusted and I had replaced them the first year and caulked each screw.
There was no sealant behind the strips and moisture had spread from the screw holes.
I am very discouraged about the assembly of these units.
Made to last a few years. But with redoing the strips have vastly improved my unit.
So recaulking from the outside is not enough if the strips are not installed properly.
My neighbor was an RV assembly line retire and was surprised at the assembly problems on my unit.