You're on the right path with record keeping. My numbers are in signature. I have not found others doing better, but the same with '03-'04 CTD's, also truck drivers. One is a man (JIMNLIN) from OK, the other (GoFastMan) a car hauler from AL or MS. Both report the same hwy mpg, overall. Mine is dead stock, except I now have two mufflers on the factory diameter pipe. So look for others with your spec truck who are doing better (and live in similar climate/topography) for a goal to achieve. My education came primarily as a truck driver (though I have always driven for economy). It's more an attitude than anything else.
Obviously, a lift and non-stock tires hurt. And money spent aftermarket is meaningless without a baseline. So get a good record book:
I use a bound WILSON-JONES Accounting Ledger (12-column; about 40-lines) that fits the console, along with a solar calculator. Takes three minutes after fillup to record the following:
(By calendar year)
Location of fillup (city, etc)
That's the first page, going across left to right.
Same line, facing page:
Trip odo reading
Avg. Mpg (from overhead)
Notes (all service)
I skip a line between entries. I do a re-capitulation at each oil change to show time/distance, average (overall) mpg, etc; along with other data that may prove pertinent. I rubber-band receipts into envelope in back (tax records), and use same to hold page open.
This is similar, but not as much room for data:
There's no point in spending money without knowing how
it is spent.
I find that 27 mph average speed means good mpg. Truck spec may vary, but from that average speed on up (and low top travel speed) is where the margin is. Never stop and never idle. Once that is learned then around town economy is easy. On the big road I run 58 mph . . . in town or out I cover EVERY mile at 1,700-1,900 rpm when solo. (I have other extended posts on "economy" or "mpg" in more detail should you wish to read them).