[elhigh]

Some ideas:

If you're ever running bobtail, some kind of collapsible fairing that would add some taper toward the rear would do good things for your aero control. But that would require some real investment to make it sturdy enough over such a large volume.

I agree that you can probably cover up the slots in the bumper. You can also go over the entire truck and probably cover a lot of those body panel separations with tape for less nooks and crannies for the wind to catch on. I know that over a chassis as big as your Prostar's there's quite a bit of chassis flex so you'll probably want to find a tape that has plenty of give, so it can move with the panels and not get pulled off.

Try covering the holes in your front wheels. I saw a quick and dirty version of that done on some big aluminum wheels in Diesel magazine; they measured a real improvement.

Time for coroplast! That big ol' chassis has a lot of big ugly gaps underneath for no reason - you can close off a lot of those and neither you nor anyone servicing the truck would ever notice.

Try fabbing up some panels - also from coroplast - to close up the gaps from cab to trailer, to make it more of one long, smoothish vehicle. If you're never pulling reefers, you don't need airflow to the front of the trailer, so block it off.