I don't know if this will help, but on my old motor home, I always wondered why the waste water drain valves where installed in such a way that I couldn't dump just the grey water. The black drain was installed in-line with the grey drain, in front of it.
By doing some crawling around underneath the rig, I found that there was ANOTHER black water gate valve installed upstream from the two valves I normally used. This extra valve had a 5 foot piece of metal rod extending (hidden) to the rear bumper.
It's a long shot, but maybe yours has a similar design.