Had a similar issue with our previous '12 36RL. Pulled down the corplast and found the drain connection had broken off. We were lucky as we had a Plant tour the next day, so I took the broken section and said "I need a replacement" they found one and I installed that evening.
Thought was that the hole in the corplast didn't line up with the drain tube, putting extra pressure, causing the fitting to break.
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