I stole my idea from Ian. Go to your local flaps and get one of those
oil change containers which has the big (8 inch?) diameter openings.
Take off the yellow cover and drop it on the ground under the drain.
When you are ready to leave put the cover back on, and drain it when
you get to an appropriate place (dump site at the campground, or
toilet, or some such). The container itself is small enough to store
under the bench in back, and it is a really fast simple method to
contain your gray water.