I would guess the cracking is due to the wall racking when driving over rough roads. As the truck frame twists, the camper has to rack, especially if the turnbuckles are very tight. Something has to give. The weak point is at the door corners. Whether or not the door has the curved corner, the frame is still rectangular. If it racks far enough, the weld is going to break. I suspect the siding covers up broken welds on most of the campers, mine included. This is also a potential problem for those with welded flatbeds. Rack the frame far enough and welds in the bed are bound to break.