crack in side of outside wall

I recently read a post on another forum detailing a repair for a crack exactly matching what you described. They had cut away the fiberglass and found that the aluminum framing between the slideout and roof had broken loose at the welds allowing flex which resulted in the crack. It was repaired under warranty so there was no indication of cost. I can only guess that it would be very expensive to repair and even more expensive if left unrepaired and ended up doing further damage.

So, yes I would run !!!! RV's already have enough small issues to deal with, you dont want to start out fighting a major issue.

Just my humble opinion