Frontal area and speed are the two great determinants of fuel consumption, all other things being equal. Selecting the "right" tow vehicle is important (engine/trans/rear-end ratio), but more important is what the tow vehicle is used for every day. If you have several children, for example, a van or large SUV may have to do double-duty. If you tow frequently, safety and comfort may trump gas mileage in importance. I could easily get 15-16 mpg towing my Scamp
16 using my Y2K Excursion with the V10. It got pretty much 15-16mpg on the highway most of the time whether or not I was towing. Towing either my 17' Bigfoot
or my 25' Bigfoot
with a much larger frontal area, that dropped to about 13mpg. That's also about what I get towing my 25' Bigfoot
with my '07 Titan 5.6l V8.
Gas mileage varies a little while towing... but in most vehicles with a tow package, it won't vary much.