The impossible: 04 Yukon XL Denali AWD

[slowmover]

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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[oil pan 4]

I loose 3 to 4 MPG when towing my 8x4 foot single axle trailer.
Its mostly added aerodynamic drag because the best fuel economy numbers I have for towing are when it was empty or only loaded with a few hundred pounds.

My friend would barrow it to tow behind his tacoma and would lose 6mpg with a bed full of wood and with the trailer over loaded with wood to the point it was up above the wooden sides and was held in place by cargo straps.
I invited him to try and break it.

We both drive our vehicles 55 to 65 mph.