Search on the Internet for '12 volt side of life' - it will be a good introduction to battery care and maintenance. Then take your battery to an auto parts store for testing, but chances are that it's shot now. Once a battery is deeply discharged it suffers damage, and if it is really, deeply discharged it is likely useless. If you have to replace it, as others have stated, ask about a true deep cycle (not a marine battery, which is a dual purpose)
- Randy
1975 Westfalia Campmobile "Backroads Wanderer"
2014 Rockwood A122 hard side popup