How to keep gray water from coming up into tub?

What the heck do all the tent cameras do when they wash their utensils, plates, canteens, whatever,???WHat about when they clean themselves?? They let the grey water flow down hill.... Its only GREY WATER after all, NO sewage.... DO I drain it on the ground, I've done that once or twice but MOST of my camping here on the East Coast is in RV Parks with full hook ups or dump stations...