truck gas mileage.

I'm considering buying a new tow vehicle, and am looking for info on gas mileage...both towing and not towing. In my 2000 Grand Cherokee with V8, I get 15 mpg in the city not towing. Towing I get 6/7 mpg. I tow about 10 miles everyday, and will be traveling about 120 miles or more about 2 or 3 times a month. I am thinking that a savings in gas mileage and less wear and tear on a bigger vehicle might be worth the extended loan period. Any input on your experience with gas mileage while towing and not towing would be greatly appreciated. Right now I am looking at possible an Avalanche or an extended cab truck (possibly a 3/4 ton).