New Brake Pads -Now Terrible Gas Milage

you are probably correct. sounds like the pads are dragging. who did the work?

 

Battery issue?

 

How many miles have you driven since the brake repair work was performed?
The poor fuel economy might be due to the rear brake shoes being adjusted too tight.
The diagnostic trouble codes logged by the skid control ECU need to be retrieved in order to say more.
In the meantime, make the Prius READY and look at the inverter coolant reservoir to see if there is turbulence in the fluid. If that is not present, the inverter coolant pump has failed. (That would not be a reason for poor mpg, but would be a reason why the skid control ECU would turn on the warning lights.)

 

If you are near a low-traffic road where you could (safely!) drive ten or fifteen minutes with no use of the brake, leading to a place like a big empty parking lot where you could safely come to a stop still with no use of the brake, that's an easy test for any dragging brakes. All four wheels and rotors should feel cool to the touch (you can reach through the wheels to feel the rotors). Don't just reach and grab, let your hand approach slowly. If you feel sizzling heat radiating out from eight inches away, you learn what you need without actually searing yourself.

For coming to a stop without using brakes, I generally turn into a big empty lot and make a wide circle in B mode, shift to R below 4 mph or so, then just stab the brake at the instant the forward roll is changing to backward, and park. Stabbed at just that point, the brake will not add any heat.

It's worth reconnoitering ahead of time to find a suitable route ending in a suitable lot where you can safely do this check.

-Chap