Ed
Not always. My black water tank has the three inch pipe that runs about 5 feet, to a valve. Now when I start using the potty, the first fluids in, do nott magically set in the tank, but travel all the way to that valve. I would not think 5 gallons of water in that pipe would be too far from the mark. Now solids, those will pretty much set on the bottom of the tank, until the pipe is full, and the tank itself begins to fill.
I hadn't considered the volume of the piping as added space however it can't be considered as tank volume.
