I have a 2014 Sightseer in which the coach batteries(2 6v) dried up quite a bit. Once I filled them with distilled water, they eventually charged up to where there was enough juice to start my genera...
Did the water level drop below plates inside cells? What kind of charger are you using and how many amps does it put out? Try to equalize the batteries first if you have that function on your charger. Getting the batteries to full charge will take a while depending on your charger.