Here’s my suggestion, disconnect your short jumper between one pair, then the next, measure voltage with good meter, that will help you determine if you have a bad pair of batteries, if they all measure the same then try each battery individually, disconnect on the short jumper wilL isolate batteries, let them sit for few hours, re measure voltage, a bad battery will discharge and voltage will drop. Now if your batteries all measure well and they measure well individually, and if you have a specific gravity tester, check each cell, they should all measure close to each other and be in the white close to green if not fully charged and in the green if charged. Once you determine you batteries are all in decent shape charge them with your inverter, make sure your inverter is set up properly for batter size, charging limits battery type etc. once you have done all all that and charged all you battteries, you can perform a equalization charge which require you to monitor batteries during the charge, you will need to ensure they have adequate acid levels, remove cell caps, have a thermometer safety glasses and gloves, run the equalization charge, cells should bubble and get warm, never over 100f, it may take hours and that may clean up your batteries. If not, they are likely shot