I had the same thing on my flagstaff. I carefully cut the caulk with a razor blade and removed the grille. I could see that bolts were not tightened enough. Take a 1/4" drive socket and snug it up so the gasket compresses. Not too tight. I put the the grille back on. I did not have to recaulk because of the clean cut I made with the razor blade. You can not see the cut. It has not leaked since. It saved me a 40 mile round trip back to the dealer. It really was easy to do.
2013 Flaggstaff 8528RKWS 5th Wheel Diamond Pkg.
2005 GMC 2500HD Crew Cab LLY Duramax/Allison
Banks exhaust, AFE intake mouth piece, Volant CIA with ram air scoop, KYB Mono Max shocks, EFI Live by duramax tuner 5 tunes with turbo brake, B&W Patriot 16K Hitch