I have the NT 22 FBS , on mine there is one handle on the left of the discharge for the gray water tank and there should be another handle on the right side of the discharge for the black water tank.
One time I was emptying my tanks and the black water tank handle when pulled came ALL the way out and I was standing there with this handle with a rod attached in my hand!!! I'm sure I had that look of what the..........well the end of the rod had threads on it, so I went under the trailer and screwed it back on the valve where it came from and really tighten it! It's still there and doing it's job. IF your handle is not there for the black water tank, I'm afraid you might have lost it! OR was lost BEFORE you purchased it!
I'm sure you can get a replacement for it! Hope that helps solves you problem.