A few years back, I read up on a product that is affixed (with a double-sided adhesive tape like substance) to the back edge of each side of the trailer/fifth wheel (vertical strips placed right at the corner on each side). They are designed to break the airflow and reduce drag and the vortex that a trailer being pulled through air causes. It's been a few years so can't recall full details but they are named something like "sharks teeth" (could be way wrong on that). New product at the time, but do recall some positive reviews. Supposed to be better than traditional wind deflectors. Don't know if they are still around but google should be able to find them.
Marv