I have never done it, though I have seen facilities that were apparently designed for hose-free dumping. I would think proper etiquette would be to hose down before and after, to keep the yuck from soaking into the concrete in the first place.
About wasting gray water. I agree in theory that it could be used to water trees and etc. In practice, I would be afraid of the policy. Some people use black water chemicals in their gray water to keep odor down. I don't think the trees and shrubs would like formaldehyde very much. In addition, I would think RV gray water, stored for a while as it is, could be growing anything you could encounter in black water.
Matt B