We don't let grease get into our gray tank. We wash vegetables and wash dishes, which might have traces of grease but everything is attempted to be put into containers and in the garbage. Almost all our gray water is soap, body wash, shampoo, detergent and water softener and a little bleach.
I wouldn't mind a bit helping to water some campgrounds vegetation, like we did in Big Bend area with it though it takes a bit of doing. Could have used the sewer hose and modulated the gray tank valve, I suppose.
We have come across where people dumped their gray tank, hopefully that was all, in our reserved spot and nearby and it stunk to high heaven.
So we use the sewer system like it was designed for, and the gray water tank flush of the hose and fill and rinse again leaves the hose spotless and smelling pretty and no chance of dandruff developing.