Gas mileage dropping on my 98 5.7L

[95 S_Trucker]

I would say that oxygen sensors and coils are not regular maintenance items. There's no sense in replacing parts when you don't have to.

The problem with the old style fuel injectors(poppet style) they will stick and gum up. They will cause hard start issues from them dripping fuel when they shouldn't. They also will cause performance problems when the poppets don't open the entire way.

See if you can use a scanner that will show live data, Check fuel trims, both O2 sensor voltages and MAF(g/s)

You can do an injector balance test to test the injector assembly.

It sounds like the problem on your truck isn't quite bad enough to turn on the mil.