Some measurements will help. Disconnect each bank ( just one link in the middle is easiest) in turn on a sunny day after they have reached float voltage, and measure the open circuit voltage of each battery in each position in each bank. Measure after the bank has been isolated for 10 minutes, so the batteries can reach some equilibrium. This will tell you if there is a single dead cell in the pack, as the voltage will be lower than the rest of them.
As well check the voltage across the inverter battery terminals, to see if there are poor connections losing power, and to see that the low voltage cut out agrees with the voltage reported by the inverter.