Horrible Gas Mileage on a Rav4 2010

Averaging out 13.8 mpg in the city, fueling up every 4 days half a tank. I’ve owned the car for almost a month using it to travel back and forth from work. Distance is 6 miles from my house to work. I also drive from here and there to target or walmart but nothing crazy in my perspective. Thoughts on this because even though I’m new car owner I feel like gas aint lasting to long. I have been doing research online and suggestions are spark plugs and oxygen sensors. How much of an improvement would I get replacing the spark plugs and is it easy to replace them considering im a newby. Also forgot to mention, I took it to a mechanic recently to replace my struts and they told me and my dad that I needed to replace the oxygen sensors but my dad said we would do it another day. Could it be the oxygen sensors? sorry if I wrote everything all over the place just tryna get some help. I’ve asked other family members what they’re opinions are and mostly they said that it seems normal but idk, i’ve heard these type of cars have beautiful gas milage but maybe I could be wrong