Why am I getting poor gas mileage in my wife's Sentra? on 2004 Nissan Sentra

I just purchased it used about 3 weeks ago. It has 74500 miles on it and we are only getting about 20 MPG. I put some seafoam in the gas thinking it would help. I installed a set of new correct plugs and checked the air filter and tire pressure. All good. What else can I check and possibly repair to improve the mileage?

Asked by for the 2004 Nissan Sentra
i would change fuel filter and clean the injectors, clean throttle body
Good advice, thank you. I don't know if I have the ability to do it, but I have a good mechanic that will do it. Believe it or not the 2004 Sentra does not have a fuel filter! I have read that there is a screen in the fuel pump.
Again, thanks for the advice.