You probably have stuff built up in the grey water tank.
I would probably fill and drain the tank once, then add one of the enviromentally friendly chems out there and fill you tank and let it sit for up to a week to allow the chem to help loosen whats there. Drain it of that, then fill and drain one more time to rinse. Water is the best solution.
What I do is after I drain and when we get home I like to add almost a 1/3 tank of just water to keep what might be there moist. On the black tank I do use the campchem treatment. I do that all camping season. I don't do it in the winter, I just make sure the tanks are flushed well after each use.
