I had a 2002 F-150 with a four speed transmission. It also has a 200 hp. motor. We rented a 25 ft. TT for a week in the Ozarks.
You had to adjust the gas pedal constantly to keep the rpms down. This gave a top speed of 60 mph on the flat and 50 to 55 mph on anything resembling a hill. Gas mileage was very poor.
we traded up to an F-150 e-boost engine with 356 hp. and a 6 speed transmission, with the same size TT we get 10 mpg.
Wind can play havoc with mileage. When we buck head winds 8 to 9 mpg is what we expect, the few times we had a tail wing we got about 11 mpg.
On the average trip we expect 9.5 to 10.0 mpg
