There was a recall on some of those models a few years ago, but did not include any of the Thor diesel lines. The motors have a plastic "gear" that slides into the aluminum tube. Some of the gears were slightly too small or the tubes slightly too large. The fix under the recall was to insert a short screw through the tube into the gear. It is a pretty easy fix. I had the same issue and called the company and they explained it to me and I did mine a couple of years ago and have had no issues since. I did have to reset the limits afterwards but that is also pretty easy and the instructions are available on the web- pretty sure I have seen them here.
Vicki and Mike, Aria 3901- The Wined Up Bus, towing 16 Jeep JKUR.