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So you were going "67mph on average" without the trailer and averaged 18-18.3mpg, and you were going "55-65mph max" burdened with the trailer and averaged 17.2mpg.
I would say that a major reason there wasn't a larger fuel usage penalty for towing the trailer was the faster speed you ran without the trailer in tow. It appears you were running at more than 10% faster without the trailer. Air resistance due to speed is a major factor relative to fuel consumption on the highway, and it builds up exponentially as the speed increases.
Another major factor that really helped you was the configuration of the trailer: it is not very high, so it presented a relatively low-resistance figure moving through the air. If you load the trailer with a high, box-shaped cargo that presents much more resistance to movement through the air, you likely will see a fuel usage differential that is much less kind to the wallet.
But there are a lot of of other things that could have come into play on your trip, especially the direction and velocity of the wind you encountered and any grade differences between the place you started from and the place you got the trailer.
The "wake-rider" factor mentioned earlier could help, but at the 33-foot length of that trailer, I wouldn't think the "wake" would extend far enough back to make much difference.
And don't hold your breath for that trailer improving your mileage to the point it is better when towing it than not, unless, of course, the grade is all downhill on the tow :-)
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