In watching cars getting 'watered' enroute (to my knowledge, only Superliners get watered enroute these days), there is only one water hose connected to each car and it's put into the opening marked 'potable water only'. That indicates to me that water for flushing toilets and sinks is drawn from the same tank(s) as potable drinking water. Perhaps when the Superliners were converted to not 'dump' at speed, they combined the storage tanks, or, at least, put a connection from the potable tank to fill the non-potable tank.