Last month we spent 8 nights at the Cottonwood Campground in Big Bend National Park. There is no potable fresh water hookup, nor a place to dump your tanks. All dishwashing/personal hygiene/toilet flushing) was done with the non-potable water (extremely hard water that was available for washing dishes and bathing, that you do NOT want to put in your tank). We did rinse in the shower with decent fresh water (we travel with the fresh water full). They also encouraged you to throw your gray water over the fence in the campground. We had 1/2 of our fresh water left and the gray was only 1/2 full. We now know it's possible to go eight days when needed with judicious use of our water.
Went camping with our grandaughter last Saturday/Sunday. One five-year-old managed to use up all our water in 24 hours.