Don,I'm worthless to you right now.I've been grinding trailer numbers for months now and I hope I live long enough to share something productive.
There are very few prediction tools for towed vehicle aerodynamics/mpg.
If you can get a hold of one of Hucho's books on road vehicle aerodynamics,he has a brief section on car/trailer aero.You'll also want to look at commercial vehicle aero and the section on NASCAR drafting.
Your aerodynamics will be governed by the frontal area of the trailer.
As you know,gaps cost us,and it's only recently that folks are actually 'looking' at the back of the rig.
Dig in and read all you can.I hope to have something to share sooner than later.Welcome aboard!
