Hi Jay, I reckon from your post that those tires are an LT tire and that at 42psi you're around the minimums for meeting your axle rating but I do not know the specifics for your truck. GT4 RPO code is 3.73 gear, excellent for towing and get up and go.
Twenty five years ago my cars made their most power on Shell's 93 octane gas. Today, not so much. I would recommend that you run a few tanks of say, Chevron's or Mobil's 93 and take note of your mileage, especially the second and third tank and then maybe switch to another brand if you feel it can be better. For a couple of years where I live it was Sunoco's 93 delivering the best mileage in our cars but for some reason things change. Is this a local problem or nationwide? I have no idea, which is why we have to see for ourselves where we live.
You might want to put another 6 psi in those tires too. If everything that's been changed is working good, then there's not much to improve upon here save for your driving habits and even then the motor could be just plumb wore out.
We used to pull our fuel rails and injectors and run them into jars and see that they all delivered the same amount of gas. A Tech2 can test individual cylinders via spark and fuel too.
I'm in a similar situation with my truck. While it had no official mpg ratings, it should get 14 City and 20 Highway. The only time I saw anything close to that was the first week after buying it, 17-18 mpg @ 78 mph bringing it home. Never again. Turned out to be a bad cat driving me nuts all those months. Replaced the whole Y-pipe and Cats with the Magnaflow and switched brands of gas. Funny thing, the truck gets about the factory ratings while on E85 but has a hard time getting close to its gasoline ratings. I finally saw an average of 15 mpg with 20% alcohol content and 17 mpg with 11% alcohol content and that was on Chevron's gasoline. Then I had to take on fuel quick and used Mobil's 93 and can barely maintain 16 mpg on the highway, let alone combined city driving. I also run AT tires @ 50 psi and have a 3.42 gear with a 6-speed transmission. I've been told the AT tires lose 1-2 mpg over highway tires. My only modification is a mild BlackBear tune with AFM turned off so that should be a wash.
So if you can get her back up to around 10-11 city and 14-15 highway, call it good.