Since you have power at the battery, have you checked under the hood at the "jumpstart" terminal? Check it with a volt meter. Should be power there since it's a straight link to the battery.
I seem to remember someone a while back having problems similar to yours. Their battery was charged, but the connection up front was bad. In fact, I think that persons front terminal basically broke off, or was about to.

 

Ok, there are no chewed wires and just for ****s & giggles I load tested the battery again it is fine, thanks to later posts for some more useful info. I have 13 volts from the battery, 13 volts up front across the positive terminal on the under-hood fusebox and the strut tower. What is the maxi fuse for exactly, it looks like like it should cut main power but I get 13 volts at the fusebox even when I pull it out. I checked the maxi fuse and all the other fuses for continuity and they're good. The only thing that will power up is the CD player to eject a disc. What could shut down a vehicle's entire electrical system like this??? I've fixed cars for years and I used to be an aircraft mechanic, I've never seen this

 

I'm really sorry that your vehicle is experiencing so much difficulty. I would like to get involved and lend a helping hand. Could you please private message me your name, address, phone number, vehicle identification number, and current mileage?

Thank you,

Jessica L.
Chevrolet Customer Care

 

The maxi-fuse is actually the "battery voltage sensor". It provides power to the computer, basically.

Only thing more I can think of is the connections between the halves of the fuse box, or the nut that holds the red wire to the fuse box. Or the BCM just decided to die because it was lonely.

 

Mine broke off had to use a sheet metal screw and a piece of copper under the the bolt to the sheet metal screw it has been several years and works great.

 

I was wondering about the connections inside the fusebox. I was kinda reluctant to pull it apart because the electronics are so complicated, I didn't want to make it worse. The red terminal doesn't seems to be broken like I've read about & the nut is tight. LOL, the BCM or ECM dying because it's lonely (or jealous of a new vehicle) is something we're hoping against. I'll have my sister contact Jessica from customer care in the meantime (it's actually her car) and see what they're willing to do