Converter size should be 30% of battery bank C/20 Amp hour capacity
Example, 100 amp hours (C/20 rate) = 30 amp converter (so a 35 is what you would get cause,, Well, 35 you can get)
Battery size Type and capacity chart (Recommend upgrade by the way)
Type is "most common in RV use"
Group 24 Marine Deep cycle 75
Group 27 Marine deep cycle 95-100
Group 29 Marine Deep cycle 100-105
Group 30 marine Deep Cycle 130
GC-2 (6V) DEEP CYCLE 210-230 amp hours (You use two in series, voltage goes up (12) Amp hours remain the same) This is the recommended upgrade to if they fit.
A GC-2 fits a Group 24 tray but is taller
Total your amp hours (IE: A pair of GC-2s would be perhaps 220, multiply by 0.3 and you get 66 amps. So a 60-70 amp converter is good.