I had a similar problem with my shower door a few years ago, and it turned out to be a busted screw in the lower corner of the door. The screw was run into the aluminum frame channel, so only about 2/3 of the screw was engaged. Either the door was heavy enough to break the screw or somebody took a ride on the door once (bought mine used too).
Have you taken the door off yet? You need to get a look at the bottom hinge area. You'll probably need a special driver bit.
2007 Coachman Freelander 26' w/ E-450 6.0L diesel
1982 El Dorado Bermuda 27' w/ E-350 460 gas (sold)