I have a 1996 5spd 4x4, with 31x10.5 BFGs, Magnaflow exhaust and high-flow cat, and consistently get 20 mpg. I have gotten as high as 22 on the highway. When I haul a slide-in camper its more like 16. I also drive like a nun. Recently, my mpg dropped to 18. I cleaned the air filter (I have the Downey set up) and the throttle body. After looking at this link
I decided to clean the MAF. On my truck though, the MAF is a solid "vortex" style unit and is not serviceable as in that link. Maybe I am missing something there, but its not clear to me how oil from the filter would affect the MAF. In the end, I have found keeping on top of the air pressure in the tires to make a big difference. This summer I plan on switching to manual lockouts in the front and will be curious to see if I get any mpg boost.

Good luck