Wsquaredodie, you seriously never got more than 15mpg? While I don't have the same engine as you, and I'm not sure what maintenance you have done, I have some recent experience that improved my 3.4 performance and it may help for you too.

When I took my induction apart to do the valve adjustment, I did several other things that I am convinced helped a little.

I separated the Idle Control valve from the throttle body and cleaned all the goo off of both parts.

I adjusted the throttle cable so that it only had a hair of slack. This took two turns out of the cable mounting bushing. I may not have been getting full WOT enrichment before.

Another thing I have learned over the years is that a bad oxygen sensor won't always throw a code and cause the check engine light to come on. A colleague of mine had a poorly responsive engine and wouldn't pass emissions. If you have a voltmeter, stick the probes into the ECU connector from behind, (where the wires leave the connector) and you can see the voltage while the engine is running. You should see something that fluctuates between 0 and 1 volt. If it's steady or not crossing the 0.5 volt mark there is a problem and you are probably running rich.

If you are only making short trips with your truck, or driving around town, this might be normal fuel consumption.
On the flat highway I can get 400 miles out of a tank if I don't stop. Going to the village and back, hauling trash, etc. I get about 15.

Iceblue, have you noticed that your mileage changes depending on where you get gas? I believe the fuel sold in high altitude locations is formulated differently.