Sounds to me like one of the batteries is either completly dead, and I mean DEAD, or shorted. You need to pull both batteries and have 'em tested.
Next: If they need to be replaced,, Remember Amp Hours, epically usable amp hours, are good things.
GC-2 Flooded wet cells (Six volt so you need two in series) 215-230 amp hours
And as these are deep cycle you can use up to half.
Group 24 Marine/Deep cycle are starting batteries, You should not use much more than 30 percent of their 75 amp horus (or about 25 amp hours each)
Group 27 is just under 100 amp hours, some brands are 100 amp hours
Group 29 is just over, likewise
Group 31 is 130 amp hours
Again all these are normally Marine/Deep cycle so limit your usage.
OPTIMA: multiply above by 60% (0.60)
Just for fun.. 12 volt 4D, you can get 'em in deep cycle, 230 amp hours, just like the GC2. That is one battery.
Except for the golf car batteires (Which become a 4D when properly installed) all figures are for a single battery
