Hmmm.... If you cannot get the clips into position it probably means the valve is not entirely closed. It may be sufficiently closed to keep black tank goo from coming through unabated but still... it's probably not closed.
If you have a black tank flush connection you should connect a water hose and let the tank flush for an extended period of time. By that, I mean let some water flow into the tank with the valve closed, then open the valve and let the water run. Open and close the valve several times and you should be able to dislodge the offending item (probably toilet paper) from the valve.
We recently had a similar problem and with about 15 minutes of filling, dumping, flushing and gently opening/closing the valve we solved the problem.
P.S. on edit I see this is your first post. Welcome to Air Forums. There are lots of people that will have good ideas for this problem and any other items that come up with your Airstream. Here's hoping you have lots of good times with your rig!
Lucius and Danielle
1992 29' Excella Classic
1996 GMC Suburban C2500 7.4L
2005 Chevrolet Suburban K2500 8.1L
Got a cooped-up feeling, gotta get out of town, got those Airstream campin' blues...