gas mileage dropped from 31 to 24 to 22 ,mpg. Happen to anyone else? on 2008 Scion xB

I had just over 60,00 miles on my xb and the mileage went from 31mpg to 24mpg and then to 22mpg I have always changed the oil on time or before and I always use the best oil, also had put on computer at dealer and had them change tranny fluid and spark plugs. I always put toyo tires on it and rotate regularly. I travel from Oregon to Louisiana at least 2 times a year sometimes more.

Asked by for the 2008 Scion xB
I don't have my workshop manuals with me. If that engine uses an airflow sensor on top of the air filter housing held on with two screws that type of air flow meter on Toyotas used to fail, set lean trouble codes result in loss of power and lower fuel mpg. With the sensor cold try CRC product # 05110, (any auto parts store) don't use anything else it will damage the sensor. A scan tool is needed to really test the sensor air flow.