If you have cycled those batteries much I suspect you have had most of their life,
If you don't have a hydrometer, try to disconnect each string at a time so that you can look at the volts after standing for a few hours.
Any large variation in the cells of each string will indicate the sick ones.
For now you may be able to get all the sick ones into one string and concentrate on them while you use the rest.
When fully charged and left standing for a few hours you should see about 6.4v per cell.
Any below 6 will be very sick and may be draining the rest.
It sounds as though you have little facility to charge the things to a good equalising voltage of at least 30V per string.
Most inverter battery chargers perform poorly from small generators. You must somehow manage to keep the charging current up by altering the settings or something to get up to that voltage.
Watch the individual cells in each string, a bad one will not respond and will drive the rest high. Any cells that refuse to respond fairly quickly should be dealt with individually but you don't seem to have means to do this.
Equalising should be done often enough to keep cells in step, when left too long you need a crude 6v unregulated battery charger that can keep amps into the sick battery long enough to see if it will respond.
Anything that will not make 6v on a long charge will have shorted cells.
You may end up with 3 good strings if you are lucky, but luck never comes my way.
Get that hydrometer as soon as you can and monitor the sg at least once a month and work on any slow cells.
Flux