If you like to do your own maintenance buying a older truck is a great option. There are a tons of great cheap trucks out there. Here is my tow rig:
The 88-95 Chevy/GMC half ton pickups make great tow rigs. Those trucks came with a 5.7 TBI which was designed for low end grunt and fuel economy. Peak torque is 300ftlb at 2000rpm with most of that available at idle.
My truck is a 1992 Chevy C1500 purchased for $2000 from the original owner. The engine is a Chevy small block so parts are cheap and available anywhere. I have towed over 25,000 miles with it without incident. I typically get around 14-15mpg towing a car on an open trailer. Since it is an older truck I have done a lot of preventative maintenance on it. When I first got it I spent around $200 to replace every hose and belt along with the clutch, water pump, etc.
If you are getting an older truck keep your eye out for manual transmissions. A manual is much more reliable for towing and will give you a 1-3mpg improvement over an auto.