Update:
There's a seam in the pipe somewhere near the a/c duct. The seam is 7" down from the top of the pipe and the pipe definitely runs thru the duct and I can feel cool air coming out of the vent. I believe what's happening is the a/c is pushing cool air up and down the pipe slightly pressurizing the tank so when the toilet is flushed it pushes fumes into the camper. I can easily move the 7" piece up and down.
I think if I could pull one side of the false wall off that would give me the best access to glue in a coupling but I'm not sure how difficult it is to pull off a wall panel. Any suggestions on the best repair that isn't tooo difficult would be appreciated.
Here are pics of the cool air condensing water on the vent cover. A look inside the a/c duct at the pipe and a look at the seam 7" down from the top.
![](http://i566.photobucket.com/albums/ss103/87GNTURBOSIX/IMG_0483.jpg)
![](http://i566.photobucket.com/albums/ss103/87GNTURBOSIX/IMG_0493.jpg)
![](http://i566.photobucket.com/albums/ss103/87GNTURBOSIX/IMG_0499.jpg)