are you sure its not damage caused by the support board while off the truck? Looks like a result of a excess pressure point and not a spread load or even support. But what do I know.
Possible heavy gauge aluminum angle running the full length? Something to reinforce and spread out the pressure point? No screws... just laminate it on with polyeurathane glue. Like adding reinforcement to existing weak area. I would overdo it with 3x3 aluminum angle completely bedded in sealant. no screws.
I only say because it looks to me like a repair will just reappear