First load test the chassis batteries as they may be the primary influence on the alternator. And at the same time confirm the voltage at the chassis battery with a known accurate voltmeter. Bulk charging rates for flooded batteries are 14.4-14.8V.
Next disable the isolator so that the house batteries are not a factor and check the voltage.
It's possible that what you are seeing is OK and the voltage will drop as the batteries get charged.
If your charger doesn't charge the chassis battery then try a portable charger on them then check the alternator voltage (give it some time).
