Maybe your chevy is losing its mind
but yeah, check your converter and add a bottle of SeaFoam and or Valvoline's Complete fuel system cleaner. you might have dirty injectors and intake valves. and if thats not the problem you might have to spend some money. air intake, higher quality plugs and wires. better cap & rotor. synthetic oils in engine, transmission, and rear axle. run with higher tire pressure. my truck was getting around 15mpg new. i switched to full synthetic motor oil and full synthetic ATF and put in some nice champion spark plugs, now i get around 19mpg. a build-up of the simple things makes a difference. good luck.