I discovered a slight leak in the closet of my 1984 Toyota Sunrader 18'. It was dripping down the vent pipe that goes down to the black water tank. Turns out there's a water line running alongside that vent pipe, and I traced it back down to where it attaches to another water line under the fridge near the heater. Fortunately, there's a valve where it attaches, and I was able to shut that off.
My question is: What is this water line's purpose? Why does it even exist? Is there any harm in always leaving it turned off?