I have a 21ft manual on our Dutch Star, and I had a similar problem. The problem was the handling strap. For convenience, I had moved it along to where it was just about 6ft from the end with the release lever. But this put unbalanced force on the roller tube and caused the frame members to skew out of alignment. When I moved the strap back to the approximate center the problem resolved. I'm not sure to what degree your 12ft would be vulnerable like this, but it's worth checking.
John & Diane, Fulltimers. RVM103 NHSO
On the road since June '12 with Lincoln, the guard cat.
2002 Dutch Star 40, Freightliner, Cat 3126, 2004 Element