Big rig trailer folding boat tail I think
 

Seen this truck running around for a month or so then got it parked at a store down the hwy. also had some wind deflectors under it.
Any ideas?

Box cavity tail, does the same thing as a boat tail, in essence, making the wake of the trailer smaller. I've seen a couple of these around---that one also has the underbody wings which should help with FE even more. I've also seen rigs with flat wheel covers to take care of the deep dish.

Pretty cool, I wonder what kind of numbers he's pulling with that setup?

I also seen a cloth like one