As far as on the road or in camp the best assumption to make is that all of your waste tanks should only be emptied into designated dumps.
There are some camps that provide special drains for gray water only. These are usually intended for tent camp type low volume use.
Some camp areas may allow gray water dispersal in bookdocking type areas but you should make sure to check the rules first.
When it is OK to disperse gray water, do so away from any water source and only in amounts that will not show after a few minutes and not cause visible runoff.
One camp I was in had rules against gray water in a county that allowed gray water dispersal. This was because some big RV's had accumulated 40 or 50 gallons of waste and just pulled the handle on the way out of the campsite. The host told me it was ok to water some of the sagebrush, though.
There are some ideas and links posted at
(along with a picture of a prize winning Airstream gopher hole!)