Another answer to the same questions.
Applicable to answer (could be modified to solve, tractor/trailer gaps) trailer boattail aerodynamics, this relatively inexpensive & easily deploy-able idea may work for most aero-improvement situations. Could be retro fitted to many older model designs & actuated by operators' air systems.
FuelSaver by ATS™
Besides offering tractor boattail form when not pulling cargo, I could imagine the side bags at full pressure to seal tractor/trailer gap. There is corresponding electronic valving controls releasing pressure on the inside bag when taking tight turning maneuvers, re-inflating when in a straight line configuration.