Recently when I went to drive my car to work it was completely dead, nothing happened when I opened the door the dash did not turn on and when I inserted the key nothing happened and obviously I couldn't turn it due to lack of power. Thinking it was dead I got my multimeter and tested the battery, read 12.7 volts. A little confused I figured I should try to jump it anyways. I hooked the jumper box up to the little red post for jumping and grounded it to the stud near that, with the jumper box it fired up just fine and seemed fully functional when running. This battery is less than six months old but I took it out and charged it to see if that would help. When I put it back in this morning I was able to start the car (battery at 12.8 volts) once off the battery but once I turned it off it exhibited the complete lack of power from earlier despite the battery still being at full voltage. I really don't know what the issue is and all the other stuff I'm seeing online doesn't appear to actually be my problem. It's obviously not any of the starting circuit because my entire car isn't getting power and it fires up off the jumper box. Please give any advice you got, I'm at my wits end with what the problem is.

No red sign on the dash saying there an issue correct ?
That would be the alternator.
Did you clean the cables and terminals?
Do you have a battery tester? Where did you purchase the battery?
Take it there and have it tested out of the vehicle. Sounds like a bad battery.
Low Cold Cranking amps, CCA
Can still be 12 v and not enough juice.
OR you could have a parasitic drain.
Most common is the seat modules for electric seats.
That will drain the power overnight,
Lots of threads on that utilizing a relay so they turn off when the car is turned off.
Or jusrt replacing them which is the more expensive option.
Other parasitic drains can be the alarm module battery but that is slower.
Not overnigjht.

Try disconnecting the seat modules with the battery fully chargged, and see if it starts the next day,
Again lots of threads on that. I don't have power seats and one less problem to deal with.

But your situation, one start and then not sounds like the battery,

No light on dash, alternator is 13.8v terminals are all clean. The charger I hooked it up too is supposed to be able to identify a bad battery and it didn't say it was bad. It's a battery from Walmart about two months old and made in Germany apparently. I doubt it's parasitic, the current in the system when the car is off is negligible. The battery is fully charged now and I don't even get the dash lighting up when the door is open, completely dead.

How did you unlock the car?

If it isn’t a corroded battery or ground connection then maybe a SAM?

Recent rains?

I got lucky and hadn't locked my car when I parked it prior to getting in. If it's a sam then wouldn't that mean the car doesn't start at all?

Use some fine emery paper to clean the inside of the battery terminal, as well as the battery terminals themselves..
the metallic interface between the two surfaces can do weird things some times, I had a similar experience a long time ago..