When I had my travel trailer, I dumped the grey water out onto the grass sometime when I was boondocking. I have also been known to dump my entire black water tank all over the Flying J parking lot. Accident of course.
But on my bus, I'm installing a 95 gallon fresh water tank on one side, and a 95 gallon black/grey water tank on the other side. It would stand to reason that when I'm out of fresh water, it's time to dump the nasty stuff, and refill the fresh. nice and simple!