I think what you are seeing in all of this data is that it takes about 14-16 mpg to drag a GCVW of 8000-9000 lbs with a frontal cross-sectional area of about 4x7 feet. I don't think about it as "what percentage of gas mileage will I lose" so much as "what will the end mpg be dragging the trailer around." Notice that the full size pickups and SUV's like Expeditions wind up with very similar mileage to the Explorers and the Mercedes SUV despite having very different starting points without the extra drag of the trailer.
This is still outstanding for pulling a trailer around compared to a full size trailer; poke around rv.net and you will see threads talking about 9-11 mpg, which is what you get when you are dragging an 8x8 box through the air at 65 mph.
I actually saw an increase in mpg with my Expedition when I went to Co Springs to pick up the TM - about 14.1 mpg going down there and over 15 mpg returning. However, that was 75 mph going down vs roughly 60 mph returning, and it's 1000 ft higher in altitude down there. So I'm sure my end mileage will be similar to what's been posted; 14-15 mpg with the trailer and 16-17 mpg without it.
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