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For a travel trailer, deduct 40% from highway mileage as a rule of thumb. For an smaller, more aero trailer, I'd think 30% is do-able. Much depends on driver skill, not just travel speed.
Lower is better. Narrower is better. Total frontal area is the main problem. Radiused edges -- all of them -- is a big deal.
See the U-Haul 12x6x6 trailer as an ideal example. I was able to keep mpg at 78% of normal (behind truck in sig).
And tandem axle tows more easily . . a bit more stability.
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