There are many things that can contribute to low fuel mileage, tire pressure, alignment, plugged air filter, brakes dragging, etc other than just engine issues. Unless you check all of the possible culprits you may be chasing your tail.
If you jack up the truck and paint a line down the center of the front tires, then scribe a line, you can check your toe with a friend and a tape measure. Checking the air pressure won't cost you a dime. I know you say you don't have any symptoms of a plugged cat, but idling around town it may 'run' decently at low RPM. Get it out of town and step it down for a while, if it starts to choke and want to stall, then you may still have a converter issue.
What does your air cleaner restriction gauge show? Do you experience poor starting at all? ( IE: possible poppet issue, dribbling fuel in to the intake )
