I don't think there is enough information yet to confirm you have dead batteries. First a few more details would help to determine this and a few comments might help you understand what's going on and maybe help extend your battery life.
First, how long since you have synched the battery monitor
? The % state of charge on these can be very inaccurate if you have not done this recently. The best, most accurate way to synch the monitor is to fully charge the batteries and manually synch, which leads to a second question.
How long since you have fully charged the batteries AND how long since you have equalized? You are in a classic
, off the grid cruisers situation which typically cycles the batteries from 50-80% or something similar. Day to day this is the most efficient way to use and charge batteries but to keep them healthy you should try to fully charge them every couple of weeks or so and equalize them every couple of months or so.
Are you also aware that the amps showing on the meter may be going to the battery but not all of that converts into charge in the battery. There are inherent losses in the conversion of amps in to the chemical recharge so if you read on a monitor 100 amps out and 100 amps back in every day, you might only be getting 90 amps of recharge on the battery leaving 10% short every day.
Voltage can be a good indicator of state of charge used correctly but in most real world situations it isn't practical because to be accurate it must be measured with no load or charge on the batteries and the batteries must have set with no load or charge for a good while for the voltage to settle. So under what conditions are you measuring 12.1V? If you have the fridge and a few lights pulling power and read 12.1V then your batteries could be fine.
So before you get too worried, find a way to top off the batteries. Pay for a slip for a day or so to plug