Friends had both slides of their Front Livingroom with that problem, and that's where the water was coming in, on the top side of that 4" Trim piece.
Their's went to the Service Center where they pulled both slides to replace the floor.
There is a U Tube around that details step-by-step how it can be done.
Michelle & Ann
2018 Chevy 3500HD Crew Cab High Country DRW, D/A, 2016 RW39MB, Dual ACs, Auto Level, Auto Sat Dish, Stack W/D, King Sleep #, 17.5" Sailuns w/Disc, MORryde IS & Pin, Comfort Ride Hitch, 5.5 Onan, Res Fridge & Induction Cook Top