I too have had the problem with my 2007 391. The design is bad. I've looked at another brand of RV using the same step mechanism and their connection to the drive motor was a much thicker curved arm. I noticed when I stepped that the steps didn't give like mine do. So I looked underneath and saw the difference. When my bolt breaks again (I have a spare), I'm going to try the arm.
In the meantime I always use an adjustable support to keep the steps from flexing and stressing the bolt. Makes the steps solid feeling too. Plus, if the bolt breaks I won't tumble. Doesn't mean the bolt won't break but it will delay the problem. I also skip the first step reducing leverage on the stair mechanism.
Lippert (the guys who supply our levelling system) now owns the stair stuff and supplies these parts. If someone has a broken bolt I would suggest talking to their tech support about substituting the "curved arm". Let us all know if that's a possibility.
2007 391TS UFO Chassis