Exactly what Schelp said.
I am glad you posted. This is interesting and I always wonder how much better mpg a 5th will get than a trailer for the stated reasons...
Try to use some sort of wing air deflector on the tow vehicle. If you have a suv or a truck with a cap then the wing goes at the back and pushes air up and over the trailer/truck gap. This will help.
I have a ghetto one on mine and by the bug splatter I can tell how high up the air is sent. It still hits the trailer but is not going into the space behind the truck and hitting the entire trailer.
I plan to make a bigger wing to get the air all the way over.