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I would definitely look into the plenum gasket, as that would kill mpg. Also, it is likely that you have the "Death Flash" on the PCM, which cuts back the timing to prevent pinging due to a failed plenum gasket. Due to the reduced timing, it hurts mpg.
These are definitely not efficient engines, but you can probably do a bit better. I get 19 - 20 on the highway at 60-65 with my 5.9, although my Jeep is smaller than your truck. Too bad most of my driving is around town, where there's just no chance (12 - 13).
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