Sudden MPG drop after leaving Car unlocked.

Hey guys !

New poster here, but I've been lurking and learning, so here's the thing.

I bought my prius 2007, last year July, car was great, but it was a little bit slow in accelerating, after about a month the red triangle came out, P0A93 code, brought the car home and left it parked on my driveway at an incline, the car stayed in that position for a year (I am stupid for not thinking about turning it on once in a while), obviously last week, I try to turn open it, but it wouldn't work, the battery died. I had to open it manually, jump started it, it worked fine but as soon as I turn it off, it won't start again untill I jump start it. I took it to my mechanic, he first replaced the inverter pump, put new coolant, and then he recharged the battery, I asked him if I should replace it, he said we left it sitting for 2 hours and it held the charge. So it should be fine at the moment but might need replacement. Now a week later, this morning, I came home from the store and parked my car , tried to go out for a drive this evening, and realized I left the car unlocked, thankfully the battery didn't die. Now here's the problem, when I drove the car, I felt a strong resistance, like the brakes are always on, I checked the car,it was in D and emergency brake was off. But it felt like the car was heavy and the steering wheel turned heavily and it dosen't glide as fast as before, almost like as soon as I take my foot of the gas it starts losing momentum fast, also the car looses speed even when going downhill when I'm not pressing on the gas pedal, the hybrid battery never falls below 4 to 5 bars. In the Display, even a slight depression on the pedal shows current MPG at 10 or less MPG , sometimes even if I take my feet off the gas pedal the current MPG will be stuck a 20 something untill I press the breaks then it goes back to 99.9 MPG.

I don't know if the fact that the car stayed parked on an incline for a year is the problem or the 12V battery.

Any help guys ? I live in the east bay area so if you recommend a shop, I'll take my car there.