I have the exact same issue with my 3500 GM Dually.
It is not a TC, but a rack and pair of heavy toolboxes sitting on the bed rails. Same effect as a camper sitting there. The front of the rack has a welded cross bar, the back has 2 steel straps in an X to hold it together as a unit like a TC. Total weight about 2000 pounds.
The bed of my truck is beat badly. There are visible stress cracks in multiple places. The sides of the bed rails are splayed out to the point where the tailgate is hard to operate.
Truck is a 2007 and the entire box was replaced in early 2012 after a hard front end collision caused the entire unit to severely push forward and down, further damaging the box.
On mine, the weight has crushed the box to frame mounts to the point where the body lines of the box sit about 1 1/2 inches lower than the cab. Maybe check yours for that too.
And I don't drive like an idiot either. I am careful on corners, slow for bumps and so on.
So yours is not an isolated incident.
I think the quality of today's trucks is sadly lacking compared to yesteryears vehicles. Thinner, cheaper, less weight, less substance etc all contributes to premature failure.
