[john70t]

(IIRC for lead-acid)
12.6v, at rest with static charge removed with a brief load, is 100%.
12.2v same is 25% charged.
When cranking, battery load should stay above 12v and never drop below 9v. Electronics run at 5-9v depending.

Batteries usually last 3-5 years but a few have survived a decade.
5v is dead. Probably an internal short as mentioned.
Replace, or the alternator might get overloaded even though it supplies most juice while running.
Don't disconnect battery while running.
Even touching the terminal wires backwards once can kill the alternator diodes, which can cause a short and drain. Bad ju-ju.

No dash lights is an interesting tangent. There may be ground issues which contributed to premature failure.
Engine/transmission to chassis is the big one, but one of the fuse boxes might also have a corrosion problem. Could be the connection or inside the wire itself. Some look good but..

Finding a short in a circuit can be as easy as disconnecting the battery negative and putting an amp meter across the gap.
Pull fuses for radio/alarm and other known Keep-Alive-Memory devices. Be careful as some might require a code and OBD2 monitor cycles will reset to zero. There are jumpers for those.
Pull fuses until the parasitic draw goes to zero. That's the culprit circuit chain. Somewhere in there is the problem.