Campsite etiquette?

This is more than a matter of bad etiquette. In North Carolina, it is illegal. http://www.ncdrought.org/documents/GraywaterFACTS-103007.pdf
I don't know about other states, but I would suspect that NC is not unique in this matter. Gray water must be treated before it can be used. Since it is impractical to treat it in the RV, in NC, it must be discharged into an approved wastewater system.

So do they not allow trailer side out door showers in NC?  It looks like, technically, it's prohibited. Hopefully, the rule is applied with some common sense.

I think that there should be a variable rule for outdoor showers.  If you're svelte and a delight to look at, then the rule about shower water should be enforced only if you have the shower curtain closed.  On the other hand, if you are painful to the observers eyes, then the rule should be enforced if you don't close the shower door.  That way there will be a clear standard that everyone can follow and/or enjoy.  Just my 2₵.  

Sorry for stirring the pot then vanishing. We were in Joshua Tree for 2 days without cell coverage.

Gray valve was definitely open. I noticed it because I heard it draining. Conceivable it had been left open from before i guess. If there had been any other sign of life in the camper, or if it hadn't been 6am, I might have knocked.