My '96 ext cab v-6 got around 15ish around town, 19-20ish highway. I once--and only once got 22 on a 600 mile round trip after it was given a full tune up. I owned the truck from 54,000 miles to 135,000 miles(9 years). Obviously the older the truck got, the worse it got, but it was pretty consistant to the end.

Sounds as if the T-stat is a little faulty, replace it---factory 195 is adequate IMO.