Empty this "lly" truck runs fine, but mileage is still bad. 12 on the highway or in town, used to get 16+ in town and 19 on highway. My truck is having problems pulling 5,000 lb trailers and seems VERY sluggish. Mileage is down towing from 14 to 9mpg, so far clutch fan has been determined to be bad (temps had gone up some while towing) fuel filter seems to be fine and computer has issued no codes.

Any other ideas what the problem could be? thanks