I know it's expensive, but I would start with a new, fully charged battery that measures at least 12.1 volts with everything off.
( or maybe you can borrow one from another vehicle just for a test ? )
Then start the engine and measure again, should be 13.8 - 14.2 volts.
If not, then alternator needs attention.

NOTE: batteries that are allowed to go completely flat are often not recoverable.