2010 Mercury Milan Hybrid - MPG going down

Thank you. The hybrid system is still under warranty, so it may not be too late.

Maybe not. When your battery charge meter indicates that itā€™s full, it may not be as full as it used to be. The capacity drops over time. For instance, my IPhone charges to 100% overnight, but the full change is only 87% of the full level when new. If your charge it meter behaves the same way, your car might be using the engine a little more each week as time progresses. You said this has steadily gotten worse over time, and that is why I wonder about your battery capacity.