really bad gas mileage

I have a 2005 Chrysler T&C Touring that has been well maintained by the dealer and averaged around 17-18 mpg when I bought it in 2005.  At that time it has 35,000 miles on it and was a dealer maintained rental return.  No is is pushing 11 years old and get an average of 10-11 mpg (around town).  The mileage on the vehicle is all city driving, but has been well maintained in accordance with the schedule in the owners manual.  Why is the gas mileage so low?  I've tried additives, put in a K&N air filter, new plugs, use synthetic oil, new cabin air filter, oxygen sensor, etc.  Do I need to take it out on the highway and blow the carbon out of the engine?  Could it need a ring job?  I'm just out of ideas on how to make it more gas efficient!