This concerns me since I have the same floor plan in the Forester.
Now thinking out loud - I wonder if the sidewalls of the class C are similar to the Georgetown class A where the side walls are secured to the framing in the floor. Our class C has the same horizontal molding trim down the side. If a fastener or two in the side wall near the slide out opening broke then the support and stress is carried along the wall. The thinnest area - the slide opening will be the weakest. This might could be verified by pushing on the outside bottom of the wall. If your unit doesn't have full body paint, the vinyl in the molding track can be pulled out just to check the fasteners.
This might be far fetched but I never thought I would read about this on a class A: Georgetown side wall bolts sheared off- and a fix!
Another cause could be too much force exerted by the slide out (extending or retracting). We all operate our slides until the motor strains or the clutch kicks in to get a good seal on the gaskets but the slide motors on these units have tremendous power. A lot of pressure on the wall opening of the coach side wall.