I agree with
Mocoondo on this subject. When I hook up I don't have a water trap in my sewer hose, and the valve on the gray water tank is opened. (This wisdom of this is a subject of another thread)
When dumping, I close the gray valve and fill the tank so that I can back-flush the black tank with it. My holding tanks are set up that way. After dumping, the gray valve remains open. Yes, it's closed when driving down the road.
I've been doing it this way over 5 years now and the gray valve isn't leaking or worse for wear.
In 2013 I did replace the valve on the black tank. It developed a slow leak. I attribute it to operator error - probably from using brute force to close the valve when there was waste material clogging the valve. I no longer (brute) force the valves to close. If either doesn't close easily I investigate. When it happens to the black valve I: 1) leave the black valve open, 2) close the gray tank valve and fill the gray tank, and 3) open the gray valve and back-flush the black tank again. This usually clears whatever is causing the black valve to stick. I could probably achieve the same result with a Flush King - but I don't own one.
Below is my new black tank valve (installed in 2013). The gray tank valve (not shown) is just to the left of the sewer hose.
