Thank you for allowing me to assist you, my name is Joe and I will be happy to assist you with your concern today.
To start we need to look at your batteries as they may be the source of the problem. 12.3 volts is actually low, 12.6 is static battery voltage, each battery should be disconnected, fully charged and then put on a professional load tester. You may have one or more batteries that are bringing the entire system down, esp when not running the engines.
If a battery is found bad then you should inspect all batteries, if they are all from around the same date and lets say 4 or 5 years old then it is really a time to consider replacing them all at the same time as you do not want to wait for them to go bad.
The generator is a separate question, however, since there is an overheat light on I would start by checking the temp sender for it and the wiring.
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