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From personal experience what you should do is buy a terminal cleaner and clean up the battery and the leads. You may have to clean the terminals 2 or 3 times in the batteries life.
I replaced a battery not to long ago to find out later it was do to tarnished connections.
Start there, then trickle charge the battery and see if she has any life left in it.
A few hours after a full charge has been completed and the battery has rested it should read 12.6 -13.8 if its still in good health. 12.5 puts it at about 80% health, so on and so forth. 12.2 volt after a few hours of rest would indicate a near dead battery and in need of replacement.
Check its fluids, add distilled water after it has been charged for a while, do not dilute it too much thinking it will help matters, keep the fluid 2 inches above the plates not near the top of the vents, you do not want to dilute it and attempt to charge it normally. Its pretty straight forward really.