I suspect work truck (note mention of plowing).
Here we have what I think is an illustration of the problem of talking in terms of "MPG", particularly for thirsty vehicles. Small MPG differences mask what are actually large percent differences.
He seems discouraged always getting "around 9 mpg", then cites a best of 9.993 mpg when trying, and 9.23 mpg when not trying. That
less than 1 mpg difference (0.76 mpg) equals an
8.2% improvement in fuel economy. 8.2% is big!
Assuming 20,000 miles per year and $3 gasoline:
$6004 @ 9.23 mpg
$6501 @ 9.993 mpg
$500 bucks a year is worth trying for in my books.
I'd suggest getting a
ScanGauge and trying hard, all the time in terms of
driving technique. Then search the
list of mods - applying what's suitable for a work truck will be gravy on top of the savings from driving technique.