I owned two F150's of that vintage. Long story short, they look aeroish, but they are still full size trucks. They also were not optimized nearly to the extent current trucks are. Look at the air dam/bumper height on one of these versus a new truck - they are way higher and as a result, allow more air under the truck and increasing drag.
The first one I owned was a '97 F150 4x2 Supercab Flareside with the 4.6, auto, 3.08 gearing, and small tires (P235/70/16). On average, that truck got 17 mpg (years of data and 163,000 miles to back it up). It could break 20 and got as high as 22 mpg. You had to keep your foot out of it and cruise on the freeway.
The second was a '99 F150 4x4 Supercab Flareside with the 4.6, auto, 3.55 gearing, and P245/75/16 tires. On average, this truck got 15.5 mpg (over many years and 193,000 miles). It would not break 20 mpg, and 19 MPG was hard to get.
Add a 5.4 to those, and watch the MPG fall some more.
I enjoyed driving both those trucks. Throw out winter, and the 97 is one I wish I could get back.
Also, the life cycle on truck body styles has shortened considerably - the introduction of the 2004-2008 F150 wasn't only due to the poor crash ratings. Note that style only lasted 5 model years, and it has decent crash ratings. The next style was from 2009-2014(6 years) and now we have all aluminum bodies...