I gat about 15.5 now with the winter blend fuel. I have a DP-tuner, and I weigh about 8000lbs. Thats driving hills and backroads of VA. Not much hwy at all. And i'm not very nice to it. This is hand-calculated.
It is a crap shoot i've learned with these trucks some guys get like 12, and some guys claim 20+. Whatever it is, don't trust the overhead display....THEY ARE NOT ACCURATE especially if it has ever had a programmer or chip on it.
I wouldn't buy a truck I couldn't take home for a day or so, so I could check it out thoroughly.
Check out this thread, it may help you.
What to look for when buying a USED 99-03 PSD