Bull, question for you: Do you have an external wiper seal on the bottom side of that slide?? We have a very early 2016 39MB and we don't have an external wiper seal, and if the wind is blowing the rain in the right direction we get a damp floor in the corners of the bed slide, plus I can see day light in the corners. I've taken to wedging a section of old 5/8" hose up in there.
I've looked at new units and they don't have an external wiper, but the slide opening is beveled upwards to keep water from running inside.
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