It is a fatigue crack from lots of flexing. Think the seam where the two long fore-aft pieces come together make it prone to both flexing and weak spot. Being about at the edge of the camper means any rocking of the camper is going to flex the point.
If your body man just welds the cracks up without any re-enforcement, you are likely to have the same issue again.
Don't think you have had this problem before because the seam may have been in a different location or not there at all.
Rubber mat may give just enough buffer to stop the flexing though a piece of plywood would definitely distribute the load