Mechanical baseline is critical. And, while an aid to mpg, it is central to vehicle performance and component life.
Combined Rig RV MPG
Orbywan's threads are to the point:
Aero RV Boattail
Aero RV Bellypan
And
Skyking's design underway:
5th Wheel Design
I'd have a long look at how many miles per year, and expected lifetime use (number of nights of use) to see what benefit aero changes will make. A few thousand miles per year may make it more of a demonstration versus a $$ savings.
I believe you'll want to conduct some tests on road horsepower demand (coastdown, etc) as your trucklet is hard pressed to pull that square white box.
The best TT's are aero in shape, but also feature low ground clearance, low center-of-gravity and torsion arm suspension (independent). A sway-eliminating hitch (such as Pro Pride) is also going to be a help from the aspect of control, and from reduction of steering inputs.
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