Somewhere I read about mounting a turning vane (what was essentially a cut section of pipe) at the cab roof top & at the edged of the windshield/door junction. This is supposed to turn the air so it runs down the sides of the cab & box trailer, which you don't have, more smoothly.
Here is a link to a manufacturer of "low drag" flaps. Also supposed to help rearward visibility in the rain.
Eco-Flaps
An outfit called AT Dynamics makes wheel covers & trailer skirts. The skirts wouldn't fit your trailer, but may give you some ideas if you don't use toolboxes.
Let us know what works for you. I need to work on my Inlaws & their grain truck.
Good luck
Don