As Eldlr said towing a big box through the air is tough on milage. The two generally do not go together. When towing a pop up we did get high teens behind a 1/2!ton with a small V8. If I were looking for the best mpg I would consider a Jeep diesel Grand Cherokee towing a used Hi Lo trailer. The diesel will help on milage and the Hi Lo will ride lower so you are pushing less wind. Ram truck is supposed to be coming out with the same diesel engine in their 1500 pick up soon as well.
I do not know the cost or quality / reliability of either of these options. In some cases the net savings in fuel can be far exceeded by the cost of the vehicle and trailer compared to a more traditional setup,
Good luck,
Stu
