How to keep gray water from coming up into tub?

A tent camper dumps graywater on the ground measured in cups or a pints at a time. An RV can dump 50, 100, or 150 gallons at once. So big difference as to quantity. Figuratively viewing it in terms of "X number of 55 gal oil drums" puts it in perspective, as does realizing the graywater contents of just one such drum would weigh 460 lbs.