Power is good but mileage has never been good since I got the truck...I usually get a best of maybe 13 mpg during the summer and sometimes dip down to 10-11 over the winter.
Also I've never gotten better than 13 mpg on any longer highway trip, even maintaining 65 mph.
What differential gears do you have? ...although, even if you have 4.10's, unless you drive with a lead foot, those numbers sound too low.
As the others have said, those timing marks are a guideline only...they're meant to get the IP timing "close enough" so that, when you install a new or rebuilt IP, the engine will start and run...however, the timing won't be "right". The next step is to take a meter and properly adjust the timing...I don't recall the exact specs but they should be in the Tech articles on this site.
Any idea how old your IP and injectors are? They have a finite lifespan, so if your timing is checked and determined to be set properly, and if you don't have anything dragging (i.e. unhappy wheel bearings, dragging brakes, etc), it could well be time to renew your fuel system. Also, are you running stock tires?
FWIW with my extended-cab truck with a lumber rack (adds a LOT of aerodynamic drag at freeway speeds), 4.10's, a ZF5, and weighing just shy of 7,000 lbs, I average 16 to 17 mpg in mixed driving and, if I'm careful and keep the speed to 55, I can break the 20 mpg barrier on the highway...