First there is another thread here in one of the other forums about "WHy can I no longer find DEEP CYCLE batteries"
So the answer is both yes and no
12 volt DEEP CYCLE are made, but they tend to be expensive save perhaps for the GC-12 golf car battery
I would add another pair of GC-2 (Also golf car batteries) Like you have now.
GC-2 Pairs are around 220 amp hours (C/20 rate)
GC-12 about half that as I recall (like 125)
Group 31, also about 125=130
Group 27 and 29 just under/over 100
Group 24 75
Ang the "Group" batteries are MARINE/deep cycle, keep at least 75% full, V/s the DEEP CYCLE (Good down to 50% state of charge)