These trucks are not very efficient running errands in the city. Wife going to mall, then waiting in line to get something somewhere, taking a phone call and more idling, etc. Mine is in top form and can get down to 9 mpg the way my wife drives it.
At 250K miles, two big issues affecting fuel economy are your catalysts and oxygen sensors. They get lazy as they age and the general consensus is to change them every 100K miles with GM OE only. Aftermarket sensors do not play nice with the computer system. if you run cheap gas like non top tier 87 octane all the time, well the injectors are probably not putting out their best spray pattern. Couple tanks of E85 or tanks of 93 and injector cleaner might help.