Elliott,

When the bus engine is running your inverters are functioning to convert battery power to AC power such as you get from the generator or shore power. Your alternator supplies charging current to the batteries so their DC voltage is maintained, and the inverters are pulling from those batteries to provide power to your critical house circuits that your converter deemed necessary.

Obviously you must make sure that all your cable connections are clean and tight. Poor or corroded connections cannot deliver the required voltage to the inverter. If I were to start analyzing your problem I would start with the cable connections.

But further information is required. For your inverters to shut down when converting DC to AC voltage high voltage could be the culprit, but so will low voltage. My first question is if the tech measured the voltage output from the alternator and then measured it at the batteries and then measured it at the DC connections at the inverters. If so what were those voltages? Do you have a meter and can you get those measurements? That will be the starting point. I think you problem can be isolated and resolved by methodically doing the diagnosis. I am concerned the tech may have shot from the hip without doing a good analysis.