[Christ]

That's the cap I was talking about originally, that has a step up that might match up closely w/ the trailer.

Is the trailer wider than the truck? If you can find some way to trip the flow off the sides of the truck outward, making it rejoin as close to the corner of the trailer as possible, that will be in your best interest, so that the air doesn't get stuck in the vacuum between the truck and trailer as it comes off the corner of the truck. To that end, some spats at the rear of the bed/cap down both sides, just enough to kick out the flow a bit, might help.

If you can take a pic of your rig, a clean pic from about 20 feet back of the front of your truck w/ the trailer attached would be perfect for a reference view.