One thing that I don't see mentioned here. We have been using our large Thetford for many years - but only for gray water. It is just the two of us, and it would take a very long time for us to fill the black water tank. We deal with the black water when we get to a dump station. You may already know this, but you can leave your gray water valve open with the tank attached. You can't leave your black water open. The solids will just hit the bottom of the holding tank and only the liquid will drain.