Here in TX, it is OK to dump grey water, provided the landowner gives permission. However, I do the old hiker thing of "pack it in, pack it out". Grey water does smell, and thanks to the slobs who dump black water almost anywhere, any water coming from a vehicle is going to earn a call to the county sheriff.
A few years ago, when I was shopping for a used trailer, one seller had a system set up where he could hit a remote on his TV, which would open the grey and black valves while he was driving. He managed to score a $3500 fine when he let loose, not realizing a Texas DPS officer was behind him. So, after paying the fine, this guy just dropped the black tank, letting anything from the toilet just fall to the ground.
Needless to say, I didn't buy that TT, and I beat a path out of there as quickly as I could. I didn't want to tell him that he was pretty lucky to get off with just a fine, especially if he managed to get black water on a police vehicle.