Tarnold, I experienced something blocking full closure of the black valve for the first time on this last trip. Dumped the tank on full hookups before leaving, and the slide would go most of the way back in, but then jam. I tried running more water into the black tank and dumping again, but that didn't help. Tried manually forcing the slide in while activating the electric switch, but that didn't help either.
What finally corrected the problem was back-flushing from the gray tank into the black. I ran considerable water into the gray tank, opened the black valve (black tank empty), and opened the gray tank valve--a technique I've used in past to balance tanks and reduce my gray tank level. As soon as I back-flushed the gray into the black, the black tank slide closed fully. I then emptied both tanks. I realize I may have just delayed my issue if there is still something in the black tank that can again move to the valve, but right now both are working correctly.
No illusions that this will be of any help to you, but thought I'd mention it.
Mike