7*31 = 217 amp hours times two = 432 amnp hours.. That's a lot of 12 volt batteries.
First you have several issues you did not address.
How many 12 volt batteries do you have (Looks like 4 Group 29's to me) what size are they, How long since you cleaned the connections, many connections, what converter do you have, did you start with a full charge.
You might find that GC-2 pairs (two six volt batteries connected in series to make one BIG (230 amp hour) battery in series) work better (Well 3 of them would be 460 amp hours at the c/20 rate. or not, they would likely be less expensive (<90 each at Sam's). Or not.
Also, be warned.. Some battery makers use different formulas for calcing the amp hour capacity.. Wal*mart for example uses the 1 amp rate.. so if it says 100 amp hours, it means it can put out one amp for 100 hours.
But we use the c/20 rate, so that same battery is like 70 amp hours at teh c/20 rate.
