Gas Consumption Problem

I have a 1998 GMC 1500 4x4 with 119000 miles. Truck runs fine, starts the first time but gas is being consumed quicker over the last year, or so.

I used to be able to drive 320-360 miles on a tank. I currently only get 220-260 miles to the tank. What is the best approach to see the greatest increase in mpg? Filters, plugs, wires, timing, O2 sensor, etc?

I have recently lost my trusted mechanic and don't want to get soaked by someone not as reputable. A lot of these things I can do mechanically, however, I am not so proficient in engine performance.

Any help you can offer will be greatly appreciated.

Thank you.