What year is your rig?
This is normally a "back to the factory" repair. They have all the tools and expertise to fix this like new for the least amount of money. Get it done under warranty if possible, but get it done before your rig needs to be scrapped for parts.
IMPORTANT! FIND THAT LEAK. It may not be right over the slide. Water can run down the roof seam from the leak location and enter where it can "puddle" under the roof material.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW