I will second Shadow on that.
One note though.. Though I have a couple of GC-2 batteries and when I pull them for replacement in a month or so they will be deceased six volt batteries and it will be sixes I replace them with..
While they are installed in the motor home they are one big 12 volt battery.
With sixes you always treat them as one 12 volt battery except you can lift that 12 volt battery out 1/2 at a time..
I have read many questions about the care and feeding of sixes.. And most all of them can be answered by "They are two halves of a 12 volt battery" so you treat them that way.
Example: Can I replace just one?
Answer: Would you want a 12 volt battery that was 1/2 old and 1/2 new?