Years ago, Maybe in the early 80's someone writing an article for Motor Home Magazine did a research on Gray and Black water left in a holding tank for a week. I don't remember all his credentials, but the results of his test concluded that "Grey" water was loaded with E-coli and other unsafe germs. Most coming from the kitchen sink. Just think of what we wash there. Chicken, Meats, Veggies, etc.
"Grey" water left in a holding tank becomes an incubator for all sorts of things. The warmer the weather, the better.
The article went on to say that he found no difference overall between the Gray or Black waste water.