Have them both 'empty'. open both valves. turn on water to the galley. when water starts flowing, close valve. does water stop? If so good, if not find out why. Shut off water and drain. Next do the grey tank the same. start with valve open and start running water, when water comes out, shut valve..... this should tell you which valve is your problem.
2013 Forest River Salem Hemisphere SBT312QBUD
2001 CrewCab F-250 7.3 PowerStroke Diesel
SuperChip, BTS transmission, 6.0 Trans Cooler