I towed a UHaul
CT13 with a '92 4WD Toyota shortbed pickup with the 22RE 4cyl and a five-speed stick. As I recall, I got 14-16mpg with it towing.
A small engine that is taxed won't get much better mileage than a big engine that isn't. If you tow 2000 miles a year, but drive an additional 12,000 miles, your real savings will be in your daily driving with the smaller engine. If you only use your tow vehicle to tow with, use the one that's most comfortable 'cause it won't cost you any more towing!
I get about 14mpg towing my 17' Bigfoot
with my V6 Tundra. I get about 13 mpg towing my 17' Bigfoot
with my Excursion. Not loaded, however, my Tundra does 21mg, but the Excursion does 13 mpg. However, I did just finish a 400 mile round-trip with four adults and luggage and got 15.5 mpg!