Maybe try tipping them on their sides for ten minutes and then go upright again. That worked with my old 27s to get the SG up more.
Any voltage you have now might be coming just from the little acid they have, all near the bottom, with water on top. Since the charger isn't getting things going, maybe tipping to mix what is there will be something it can grab to and improve?
Note the highly scientific battery expertise there. Or first kill a chicken and let seven drops of fresh chicken blood fall on the positive posts of each battery--not the neg posts, or it won't work.
Did I mention we need a battery guru or two to help out here? :(