Assuming your mileage is just worse, but not 1/2 as much worse, consider that possibly the gasoline brand and grade you have purchased may now be the winterized gasoline.
I admit not being an expert on gasoline, but have read that the companies do alter the blends summer vs winter. I would assume that since I live in Florida, we may get summer blend all year.
Not saying this is the cause of your concern, metely a consideration.
Like others, almost each time fuel is added it is logged. Mine is done as a complete log donein Excell. Each row represents a date, mileage, the task, whether maintenance, a modification, or adding fuel. The rows where fuel is added, automatically calculate mpg. Any trends in a negative manner would be cause for concern. One kind of cool thing though, is if a mod is done, say adding a cat bypass, different air filter or whatever, the results on mpg are easily seen as the following rows.
Whatever it is, I hope you find the reason for reduced mpg.