Prius 12v battery draining even after replacement.

Hi guys, one morning my Prius just happen to be completely dead. I tried jumping the car and it turned on completely however after removing the jumper cables the car automatically turned off. After turning off it would not jump again. I checked the battery with a voltage meter and it was reading 1 volt. I figured the battery was dead and put in a new battery. After installing the new battery the car worked fine for 2 days. On the third morning the car was completely dead again. I checked the battery and it was only showing 6.2 volts. I tried jumping the new battery and it did the same exact thing as the old one. Engine started, but shut down when I disconnected the cables. No lights or anything obvious were left on, so I don't know whats causing the battery to drain.

After reading posts on this forum I read somewhere, on a related issue, that the car should start using only an assisting cars battery. I took the new battery out and attached jumper cables in the correct spots to the assisting vehicle. Prius was still completely dead, no luck. Any advise and help is appreciated!