[RDS in W PA]

Okay guys need to pick your brains. I have owned my 2002 F-250 since brand new. First year I owned it I took it to FLA. towing my fifth wheel trailer. Checked my mileage computer against actual fuel used for the entire trip. (Over 2200 miles) It was dead on, so I trust it to be accurate. I got 11.6 mi./gal. on my first trip down. The next year I went to FLA again. This time all the way down the best I could get was 10.3 mi./gal. On the way back I had a long uphill pull about half way home. During this pull I ran the truck in fifth gear, 55mi./hr, about 2750 rpm. This pull took about 8 minutes of steady hard running. Near the top of the hill my truck started making a different sound. Sounded like an aux. fan kicked on. That isn't really what happened but there was some sort of hum audible in the cab. From that point on the truck ran better. The throttle was more responsive and my fuel mileage improved back to 11.6 mi./gal. for the balance of the trip. This same thing has happened two times again since the first time. What is going on here? Is my catalytic converter plugging up and choking off my fuel economy and power? Could there be carbon deposits on my Turbo charger impeller on the exhaust side that I takes a hard run to heat up and burn off? Any one else have a similar experience?