Red triangle, ‘P’ lock, then nothing

Two days ago my 2007 Prius (155k) gave me the red triangle of death. It said "there is a problem with the transmission "p" lock mechanism. Park your car on a flat surface and fully apply the parking break". I didn’t drive it.
The next day I went out and started the car, the same thing happened. Red triangle. It had been raining so I didn’t drive anywhere and I didn’t troubleshoot.
Today I went out to finally work on it, and the lights started out dim, there wasn’t enough juice to fully light the dash, and the remaining fading light was the check engine light until everything went dark. Now there’s nothing.
Before the car fired, I found the following:
At first I was prepared to install a jump wire from the 7.5 fuse under the hood.
Then I was prepared to seal the back cracks near the 12v battery if water got in. (I didn’t see any water.)
But today the car is black. No lights, no sounds, no nothing.
I’m thinking of replacing the 12v battery at auto zone.
I’m visiting in Western PA, so I don’t know any Prius people around here.
Should I attempt something else first? Like a basic jump? Or is this clearly a 12v battery issue? I’ve no idea the last time the 12v battery was changed. I’ve had the car for 9 years.
I’m looking for a fix, not something short term.
Thanks!

 

OEM 12v or any AGM will do?
I heard the local dealership uses Interstate batteries and Toyota puts their brand on it.
Sounds like I can buy that battery from them and install it myself. Trying to see if anyone sells this Interstate battery near me. No luck so far.
Thoughts?

 

Bought OEM battery from Toyota. Installed it myself using YouTube video. Took me longer, but I’m paranoid.
So far so good. Car is alive. No scary lights. No wonky death triangles.
Going to seal seems in the trunk because there was a wee bit of moisture in the battery compartment.
Thank you!