Is there no problem dumping grey water on the ground in campgrounds? My grey water doesn't have food particles or grease. It's only hand soap, or dishwashing soap.
We use gray water in our garden fairly often -- we have an outdoor shower (as well as two indoor showers), and we collect the outdoor shower water and put it on the plants. No problem -- the plants seem to enjoy it.
But I would not do this in a campground (or while boondocking). Even if it is supposedly good for the plants, it is against the law and is rude to the other campers.