There is a 30 US Gal tank (40 US Gal –400 series) behind the aft cargo hold for potable water. This serves the galleys and washbasins, but not the toilets as they use chemicals. Waste water is either drained into the toilet tanks or expelled through heated drain masts.
The tank indicator (-3/4/500 version shown left) is located over the rear service door. Press to test, indications are clockwise from 7 O'clock: Empty, 1/4, 1/2, 3/4, Full. The NG has an LED panel that is always lit (below) for both potable water and waste tank.
Potable Water Quantity Indicator - NG
Potable Water Quantity Indicator - Classics