mpg question on older trailbazer

Hi guys, newbie here, stumbled across this place do to a vehicle concern,

I have an 02 trailblazer, with 140,000miles... this past summer i had new spark plugs installed, along with a fuel system flushing and filter change..
Recently i have seen my city mileage, which usually goes for 270 miles a tank drop to 200 miles per tank. (highway was about 300 and is now 270)

I checked my tires, and have no dummy lights going off...(plus recent plugs and fuel system checked...) Even tried a few additional fuel system cleaners, but to no avail. this is the inline six, not eight

Any suggestions, comments, or insight will be appreciated.

Welcome to AF, my 02 is about the same here in upstate NY, shorter trips, cold weather and the use of 10% eth. all contribute to this. Also you could do a full scan and look at all sensor inputs particularly o2 sensors.