It looks like there are design problems with trying to build articulated gap fillers that allow a vehicle with a trailer to turn corners and go over dips and bumps. I was looking at Phil Knox photo albums in Pickups and found this page from a study of truck bed aerodynamics.
Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - aerohead's Album: Pickup images - Picture
In the study, a truck with a fairing on the cab and a partial cover on the bed was almost as aerodynamic as a full boatail on the bed. It made me wonder if a Kamm back like Metrompg's and a trailer like AndrewJ's would be almost as good and a lot less complicated than trying to completely fill the gaps between vehicle and trailer. I'm hoping to build something like this myself to haul bicycles and camping gear. Roof mounting bikes clearly kills mileage.