Slide topper part came loose - what is it?

That piece is meant to prevent your slide topper awning from unrolling from wind while you are going down the road. There should be a bracket on the coach sidewall that, if the awning starts to 'roll out', will be struck by the pictured part and prevented from rolling out further. When you extend the slide, the outward motion of the slide will cause the pictured piece to be too far out to strike the bracket when it rolls around to line up with the bracket. The piece you pictured fastens on the end of the awning roller. Since you have others to look at, you should be able to figure out how to replace and adjust the part. Yours probably was loose and the adjustment slipped to a point a little too close and the piece caught on the bracket then popped the part off. If it wasn't night, I'd go out and attach a picture of mine. Mine is attached to the roller with 2 screws and is slotted for adjustment. Your wife probably saw the screws flying off. If the holes are stripped, you can re-drill and tap elsewhere since you have the slots to work with. With it off, there is the possibility of the topper seriously unwinding as you drive down the road if the wind catches it right