Gawd, I will probably get totally laughed at for this suggestion but here it goes:
Toyota Sienna minivan
If you get the correct year (2007?) it will come with the 3.5 liter V6 AND the engine oil cooler. Which means it will be rated at 3500 lbs towing. It also will have the same drivetrain as the Highlander.
Can be bought cheap used. Gets close to 25 mpg when it isn't towing. Also available as AWD if needed.