I posted this some time ago, MB owners don't try this trick or you will fill your Gray and the Galley will be empty. In the MB the tanks are not on the same level. The Galley is over the Axles in the Belly, while the Gray is in the Drop Frame section next to the main Black Tank.
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