Ignoring the high voltage / Jake brake issue for a moment I think I understand the voltages go down when the inverters are supplied with AC power.

I do not know what alternator voltage means, but regardless of where that measurement was taken it says if I am reading it properly, when 120VAC is supplied to the inverters, either from the generator or shore power, with the engine running the chassis batteries stop receiving a charge. They are at their normal voltage when no charging current is being applied to them.

Did I get that right?

If so, the next numbers I looked at were the value of the voltage to the regulator while this was happening. I did not see high voltages that would cause the regulator to disconnect power to the alternator field because the voltage was beyond the regulator set point.

Now, for reasons I have not yet fully considered, I am thinking the isolator may be the issue. If the coach were mine, I would disconnect the alternator terminal and chassis battery terminals from the isolator, bolt them together, start up the engine and generator and see what happens to the chassis voltage.

Don't ask me why just yet that I am thinking along those lines, but by doing this there is no downside, and it takes the isolator out of the equation as a potential problem if the chassis voltage remains low with the generator running.

All I can say right now is this might have something to do with the inverters being powered. Before disconnecting the terminals and joining them it would be of value to shut down the inverter battery charger functions while the generator is running to see if that has an effect.