You've probably already answered your own question but try and slow charge the battery for 24 hours, or longer, and see how it recovers. Should charge up to about 12.8v. It will drop a bit but shouldn't go down to less than about 12.6v. If you take a fully charged battery to a battery shop they can do an electrical "drop" test on it where they do a quick and dirty total discharge on it and see what the meter reads. Bottom line, buy a new battery and go from there.