Same problem with my unit
Bought new 2010 Flagstaff 8526 with the optional 15,000 BTU Coleman Mach III.
The guy who installed it did not know that the gasket had to be compressed evenly till the yellow tabs that fit inside the gasket opening touched the roof top.
He just screwed the unit down, and failed to notice that the tabs had folded under the gasket. This allowed a path to form over about a month of rain till water was leaking into the camper. (water on floor directly under the grill).
Removing, replacing the seal, and correctly installing the AC was covered under warranty.
NOTE: The "bastads" CAULKED the grill cove cover in place! This allowed the water to fill the grill before leaking. Also made removing it a PITA as the tech had to slice all around the unit with an exacto.
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