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...
You've probably ruined the batteries by letting get that low. They will need to be replaced. I did that with my first set of batteries by not keeping an eye on the water level. Now I have the auto fill system and I fill them on a regular basis when I have the motorhome plugged in.