Low Gas mileage and code P0A1F

Hello, first time poster. Mileage is about 210,000ish it's a 2006 Prius Wife and I took the Prius on a road trip from WA to UT, on the trip back we got the triangle of death plus a couple of other lights, noticed my gas mileage shot way down (like I swear I was getting like 15-20 MPG when i was getting 42-45) and the Prius battery was showing up as like 1 or no bars we ended up buying one of those diagnostic things from Walmart, and got a code P0A1F picture attached I literally don't know what the other screen is even for (the one with the symbols) we took it to a firestone and they did something though they said they couldn't work on the battery or some wire system thing? Anyway we are looking at replacing the hybrid battery when we bought it the previous owners had replaced it with a refurbished one 3 years ago and it's been 4 since it's been replaced so it might be due? (I'm not really sure) I've googled the error code but it really was over my head. I've been getting about 38-40 mpg if I go 55 mph or less and the battery seems to stay chargedish but I've always felt the battery since we bought the car hasn't been staying as charged as it needs to (again I'm no expert this is my first Prius) but when I go 65+ the battery drains quickly and my gas mileage plummets should am I doing the right thing by getting the battery replaced, or could this be a way more expensive problem? I check and change the oil when needed with full synthetic.