I'm hearing you say that at 65-mph, solo, at 75-feet behind a semi with trailer van, your highest mpg is 32.5 ?
And, pulling the trailer with bikes, your all-up weight is around 7,900-lbs?
Do we have an mpg at 65-mph pulling each type of trailer ( open and enclosed ) ?
I saw a 21.5 mpg figure, but not sure how to connect that to the different architectures.
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It's possible, even with a weight penalty, to experience zero mpg loss when pulling a trailer. I've experienced this.
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It sounds like you've studied NASA's aerodynamic truck experiments at Edwards AFB, California.
From those you'll remember that:
1) lowest drag occurred when the tow/ trailer gap was closed ( articulated gap-fillers ).
2) exit frontal area of tow matched forward frontal area of trailer ( a seamless transition).
3) NASA had a full belly pan, you'll have a full belly pan. All is well there.
4) NASA had inset wheels, you'll have inset wheels.
5) NASA dis not have wheel skirts. You could have wheel skirts ( an improvement over the NASA project )
6) NASA had zero longitudinal body camber ( pressure recovery ). Beyond your trailers maximum cross-section, you could conceivably incorporate plan-taper and roof taper, improving on NASA's design ( a fabrication complication, however, exactly what would deliver lower drag ).
7) Flow reaching your 'Dryden' tail would already be oriented towards the wake, and at lower velocity and higher pressure ( like Railton Mobil Special, MG EX-181, Goldenrod, Challenger-I, VW Flow- body, EV-1, etc. ) providing premium onset flow to the boat tail.
You're the only one who knows your requirements for ease and safety when loading and un-loading the bikes.
8) Whatever meaningful drag reduction you achieve, is going to come from a gradual, streamlined reduction of the trailer's body cross-section and tail ( given that all else has been 'cleaned up.'
9) You're appetite for fuel savings vs construction-difficulty-coefficient ( PITA) will decide your course.
