Oddly enough on my recent trip to Indiana, we used a Honda Pilot (3.5L Vtec V6) to tow. But that was with a light car and trailer combo (~3500lbs combined), but it did just fine, maintained 70 MPH uphills, with some effort of course, but it always had enough power and the right gear (2nd gear at 4K for some of the bigger ones) to get it done.
If you put some thought into your setup, a vehicle that's not known for awesome towing capability can do just fine. But I don't think you'll find anything capable of towing ~4500lbs reasonably and getting 25+mpg (not towing). Even the Pilot only got 21-24 MPG not towing. But we did get 17 MPG on at least one tank with the car and trailer on the back.
Over on the SCCA forums, there is a section dedicated to towing. You might want to check it out.
http://sccaforums.com/forums/70/ShowForum.aspx