Sounds like something is hurting it somewhere. I know EPA estimates of 28 mpg mean nothing becuase that is also average some people may get more some less. But I haven't heard of anyone getting that low. When I drive the car. Driving like I want to go fast
i still get 28 to 31 avg depending. (MPG Guage shows 30 to 34.)
How is your filter? Clean or really dirty?
Does it seem to be driving fine, i.e. no loss in power? I would pull each plug and check it. Maybe one of the plugs is going or has gone out.