Shirley, I use a Y at the campground water source and have two hoses hooked up. One for my motorhome water source and one I use to flush my black tank and any other need for water at the campsite. Until I read your post I hadn’t heard of a macerator. When I flush the black tank, I then follow it with the grey tank. I then close the grey and , with the black open, flush the black tank with the motorhome flushing system. With showers and dishes, we produce a lot of grey water so when I need to flush the grey, I flush the black also, even though it's not even one quarter full. With the flushing system we have, we can keep the black tank supplied with plenty of water.