Mark,
My insurance company said unless I could get the driver to admit and take full responsibility, then they would not pursue the truck company when I had a similar incident.
Technically, the trucker or his firm could be cited for a loose load. Why does everything else have to be covered when we go to the dump, except for the most dangerous stuff like rock and asphalt doesn't?
But again, even if you know it came from the bed of the truck and not from under the wheel (the former could be grounds, the latter is considered bad luck), proving it would be pretty much impossible...
So I think you are going to find you're on your own, unfortunately.