Arctic Fox 1150 is breaking my 2016 Ford 350 dually

Avid Fox wrote:
If the hj tie downs and resulting bed flex were the specific problem, I'd see at least some buckling at the bed's front also, but there isn't any.

The front of the beds are braced better than the rear, especially when you aren't running a tailgate to limit the twisting in the rear. Forces travel. I assure you, the damages you're seeing here could easily be (and are) originating from the front of the bed, at the tie-downs.