I know you! You're one of those people that uses your truck as though it were a truck. Practical, sensible and adult like. I agree with you. I didn't know anything about TBIs when I bought my clean and stock 1993 K1500. I 'm pretty happy with it but the performance wasn't what I expected from a Chevy 350. Rummaging through lots of bad information I finally understood that Chevy built it this way for reasons that Tom elucidated. It's a truck engine for a fairly light duty full size truck. It gives you the reliable low end torque and it is low maintenance. The worst aspect is the gas mileage.
I'm glad I didn't modify it. I don't know what the original mileage was supposed to be when new but mine gets between 10 and 12 MPG. With the current gasoline reality I'm very interested in whatever will improve the mileage.
So what can be done to improve the economy of the TBI?
What rear gear do you have?
How old is the O2 sensor?
You could try disconnecting the EGR valve for a tank or two, that may help a little.
Make sure timing is up to *****, and even add 2-4 degrees.
How big are your tires? Larger diameter tires will actually cause you to travel further than your odometer records, falsely skewing mileage numbers lower than actual.
Is overdrive working and the TC locking up?
You can save a lot of fuel if the traffic patterns allow a more gradual increase in speed.
I don't know what a 350 K should get, but my buddy's Dad used to coax 19 on long family trips out of a '73 Chevy wagon that of course had no overdrive.
I get 12.2x with my 1 ton 7.4 4.10 crew cab around town. Although my around town is semi rural so it's not the same as city driving. I would think that the 350 K motor should be able to do a little better.