The most likely problem is a flaky fuel gauge, the rest of your vehicle behaving normally, as ever.
It’s time to start calculating your mpg the correct way, by recording gallons purchased over a period of time and writing down the mileage that appears on the odometer.
Too many people think it is sound to determine fuel economy by miles per tank. This may be a simple method but there are far too many ways to get an erroneous result.
For now, at least, I will suggest there’s nothing wrong or different with your vehicle’s fuel economy.