There's a big difference of a tent camper dumping a dishpan of water per day on a bush.
RVers use water for many uses and when they're dumping it could be 40 gallons or more.
That site will be occupied by a person after you leave and another person after that and another, etc. If everyone was so inconsiderate about dumping their water think of the condition of that site with soggy soil, smells, hair, soaps, garbage.
Why can't folks think of others than just themselves?
As volunteers one of our little 'duties' once was in a national park. We had to remove the rock and soil under the roadside faucets because they were not draining. It was disgusting to have to remove the hair, garbage and toothpaste because, despite a sign posted right on the faucet of "no washing of dishes, hair, bodies or brushing teeth", folks continue to do it. We removed the debris down a foot and replaced it with new. Probably the next volunteer in that campground had to do the same thing the next year.
Think it out folks!