Our motorhome is 10 years old, we spend a lot of time in the rain soaked northwest and all three slides have leaked at various times. So have the front cap and an A/C unit. Right now I can say nothing has leaked for about two years.
Because Beavers are flat on top, water does not drain off well. (Most "lesser" RVs have slope and gutters built in!) Because of the uneven flatness and variables in leveling, everywhere there is a rivet, seam or attached device there is also the opportunity of a shallow puddle that will leak through. The answer is to clean each suspected rivet and seam as good as you can and apply new sealant. (For one slide leak I used to adjust the leveling with a slight left slope to get the water away from a seal!)
I would not use Dicor as it is self leveling and seems to stay "gummy" for awhile. Lexal is a favorite of many. Silicone is not, although I've used it with apparent success. Whatever you use, it should be UV resistant.
The hardest slide seam to seal it the inner one that goes front to back. From your picture, it looks like that is the one that is your current problem. Once I have the entire slide topper/awning removed, I can lift the "wipe seal" enough to apply sealant to it. When it was new and under warranty, a factory tech sealed one slide by fitting a piece of clear tubing over the nozzle on a caulking tube as an extension, brought the slide in and sealed it by feel as he reached between the top edge and the ceiling. His coverall sleeves weren't clean, so we had to clean the ceiling fabric when he finished!
Good luck with your repair. It really hurts to see water stain damage on our beautiful interiors when it is so difficult to keep them sealed. We spend the extra money every month it is parked to keep it under a roof. We watch carefully when it rains. And, we've spent a lot of time in Arizona the last couple years. I hear the Californians have stopped getting rain for awhile. Maybe that would be a good place to travel now.
Phone me if you think I can be of any help at all.
