An object paasing through air will create a vacuum behing it. The dimples on a golf ball are actually there to break up the vacuum created, otherwise it would fly kind of erretic like a ping-pong ball.
With that wing being smooth and extending over the rear like that creating that pocket as it does, I'm pretty sure you would increase the amount of vacuum behind the truck and dirt would collect more.
This is kind of what is behind drafting in auto racing. Get your vehicle in that vacuum pocket and let the car in front "suck" you along draining his fuel and reserving your's
With that wing being smooth and extending over the rear like that creating that pocket as it does, I'm pretty sure you would increase the amount of vacuum behind the truck and dirt would collect more.
This is kind of what is behind drafting in auto racing. Get your vehicle in that vacuum pocket and let the car in front "suck" you along draining his fuel and reserving your's