Is your Black now Full?? Sure sounds like you hooked up to the Black Tank Flush port instead of the Fresh water Fill port.
There should be no way for water to fill the toilet when connected to the Fresh Water Fill port unless something is cross plumbed.
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