On my 98 Explorer, I've had excellent results using the Super Chips Tuner (SCT). Basically bought the tuner used on ebay for $75. Then paid a guy named Doug, at BAMA chips to write three tune files for me. He emailed them to me and I uploaded them to the tuner. It plugs into the OBD II port on the truck, and gives me the options of changing shift points, idle speed, ignition timing maps, and full throttle mixture, also allows rev limiter change, and shift firmness adjustments. I love it, and for towing the pop up camper it made drastic changes.
Anyways, on previous camping trips to key west we always got 11 mpg towing the camper, and had no power above 65 mph. I told doug the problem and he wrote a towing tune file. I uploaded it, and the next trip we had tons of power up to 80 mph, and mileage jumped to 15 mpg. Without towing our mileage went up from 17 mpg- to 20 mpg. It's only a V-6 but with 4:10 posi gears and 182K miles it's starting to lose mpg's.
Good Luck