Decent, but probably should be better
20 around town isn't too bad. It's really contingent on several factors though (number of lights/stops, hills, speed limits, weather, your driving habits, status of your tune up, extra weight in the cab/bed, etc). With a stick, I would guess that you should be doing better, even on winter fuel mixtures. With an automatic around town (with plenty of hills and stops), I'm getting around 23.5 mpg. I think that your numbers should be better than mine and more in line with Brutus' summer numbers. Cold is another factor that bites at the mpg's as well. Try to avoid long warm ups and try to pulse and glide (especially as the engine is warming up; give gas in gear, and then coast). Also, if you are so inclined, purchase an Ultragauge or the like to give you realtime feedback on your driving and truck performance. Learning where best to shift, where the torque is best, and where you can take the best advantage of when to give gas and when to hold off will help.
Good luck!