[Javi]

2890RL has one black tank of 30 gallons, one galley tank of 30 gallons, one grey tank of 30 gallons, and one fresh water tank of 30 gallons.

The black, galley and grey tanks all dump through a central manifold with three separate valves conveniently located side by side.

I doubt your grey and galley tanks are connected except through the individual valves and central manifold.