I wonder how hard it would be to make an accordion connection between the cab and trailer, like you see in articulated buses. Even though the airflow would still be pretty dirty around those folds, the majority of it would have to flow around the whole thing as a unit rather than detaching and swirling around before hitting the square front edge of the trailer.
If there's one thing I've learned on EcoModder, it's that even when a vehicle company tries to make something reasonably aerodynamic, there are still lots of opportunities if you're willing to put some work into it.
