Sluggo, I know that you've mentioned this before. I haven't seen how dusty your truck gets.
I guess I lucked out. My 2014 Silverado with a tonneau cover is pretty good for dust. I haven't ever noticed a problem. That red Sierra that I had, had seals on the bottom and sides of the tailgate, and maybe I was just lucky, but that one wasn't dusty either. I never owned THAT truck without the tailgate seals, so I can't compare, but those seals seemed to do the trick.
I know too, there are usually some sort of drainage holes in the floor of the bed, near the cab. I wonder if that's not where the dust is coming in.