I would think this would be the exact reason many people here do not encourage putting anti-freeze in the fresh water tank. I think I would fill it and drain it a few times to dilute the anti-freeze. Before my Salem TT, I had a pop-up. The drain and the discharge to the water pump both came from the side of the tank, not the bottom. The drain was about 1/4" up the side of the tank, it hardly could be installed any lower, so it always left about 1/4" of product in it. The drains usually are not in the bottom, as they should be, if they were , there would probably be 1/4" lip for the fitting anyway, so you couldn't get it 100% drained either. As a former water plant worker, the solution to pollution is dilution.