Q: gas millage is low , i keep putting gas in every day i only go 15 miles from home a day, i have put $20 every 2 days in the tank

So every to days im putting gas in the truck. I bought the suv used and jabe had the spark plugs changed and oil changed . Ionly go 15 miles a day for school and work $20 gave me one line above Empty now im two lines before Empty. Whats the issue , there is no check engine light on but i smell sometimes a faint trace of gas . Please help

My car has 236000 miles.
My car has an automatic transmission.