2018 Montana 305RL - ceiling leak

[bubbapk1]

Purchased our 305RL in May of 2018. We use it occasionally and have kept it covered. We are readying it for our next trip when noticed a black substance on the floor. Tracking it to the back of the RV, we saw the that the leak was coming from the ceiling at the rear-most seam. The ceiling appears to be sagging, and the strip that covers the seam was spotted with black mold. I pulled the strip off the ceiling and water dripped down. I put a screwdriver in the gap between the seam and about a pint of black water ran down.

I imagine that this has been an issue for a while, it just couldn't take it anymore and started leaking. I am attaching a picture and will attempt to attach a video.

What I am asking is if anyone has an idea what caused this? The roof seems fine on the outside. I have an appointment at the dealer in 5 days, and would like to have some knowledge going in. Also, any ideas on how to replace/fix?

You can see in the picture that the strip is spotted with black stuff, and it appears that staples toward the back are rusting and leaving spots on the ceiling. The LED light is also removed as water came through there and made it black.