Letting the grey water out a bit to extend it's use?

I don't know how many times you would drive down the road with liquid coming out of the drain line before you would get pulled over. Too many people with cell phones.

If he's draining the grey water out on the way home what does he do with the black tank? So he gets home with an empty grey tank but the black is still full? Then once you drain the black tank you have nothing to rinse the hose out with. I'll pass.