My 2006 Duramax gets about 13mpg Towing my Coachman Captiva 265EX (Via the lie-o-meter, I choose to remain blissfully ignorant of my real mpg's).
I love the excuse that people use better gas mileage to buy a new vehicle. Even the most creative accountants will have a tough time penciling that out! A buddy at work just traded his paid off 2001 Durango for a 2013 Subaru Legacy... because his gasoline bill was killing him and what he saves in gas will pay for his new car payment.... right. I just smiled and agreed.
If you want to buy a new vehicle then buy a new vehicle. Don't try to claim that you are doing it to "SAVE MONEY" though. That is pretty much always a ridiculous claim!