Another possibility is that, like many of us, your black and gray tank valves are mis-labeled. Try opening the valve labeled black and if your gray tank drains, you'll know they're mislabeled.
Happy trails,
__________________
Bill Stuart
2012 Alpine 3555RL
2015 RAM 3500 Laramie Limited Mega Cab Dually 4X4 Cummins, Aisin trans, 4.10 gears
Andersen Ultimate 5th Wheel Connection
Previous RV's:
1992 Prowler 35' Bunkhouse
1986 Fireball 30'
1978 Nomad 24'
1963 Kenskill 19.5'
1972 Dyna Cruiser Camper on '72 F-250
A Tent
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