Everyone says that I should empty my black tank before my gray tank. Is this right?

Ready for a revelation? No! Do not empty your black tank before your gray one(s). But here’s the catch: Open your gray tank pull just a little bit, allowing some of the (relatively) clean wastewater to coat the sides of the sewer hose first. Then, close the gray tank pull, drain the black tank (turn on your built-in flush system during this process — more turbulence!), close the black tank pull, then drain the remaining gray tank(s).

Why do you open the gray water tank pull first? The relatively clean water in your gray tank “sticks” to the interior surfaces of your dry(ish) sewer hose (the “slinky”), creating a barrier. When the black tank wastewater then travels down the hose, it is likely to travel at a faster speed (there’s already a barrier of “clean” water on the interior of the hose). Following up with the gray tank water will completely clean the inside of the hose.

We know, it sounds weird, but it’s physics!