Recently I was attending a seminar and an interesting question was posed, one that I thought I surely knew the answer to. "When dumping do you empty your black tank or your gray tank first?" Gut reaction and what I had preached for years, (Yes I have dumped both tanks before!), BLACK! When I explain the reasoning that this is false you will be understand.
The speaker went on to correct everyone that shouted a resounding Black that you should actually dump the gray tank through for an allotted time to get the black plastic pipe good and we,t like a slip and slide so that any "gunk" coming out of your black tank slides right on through the pipe and into the dump station. It makes perfect sense when you think of it. Moving anything with any substance through a dry plastic pipe is going to rely on the pressure of your water moving it through. When anything with substance moves through a wet and soapy lubricated pipe it is going to be less likely to stick around and make your pipe stinky.
Brilliant! So my new routine is to dump my gray tank first and let it run through the pipe for a few minutes then empty out the black tank and then back to the gray to flush whatever is left all the way out.