Black Tank Flush Hook up seamed plugged up.

[twindman]

Most likely the flush line is plugged by the back flow device. It was behind the wall in my 2006 2980 model. In my mountaineer it was under the sink. My 2006 was fixed at the dealer - they cut a larger hole in the wall and fixed it and then covered it with a plate like thing.
Good luck.
p.s. I am pretty sure someone else will come along with more specific advice.