John,
Your coach uses a Duvac design Leece Neville alternator that requires a sense wire to regulate the charge voltage because the battery isolator blocks battery voltage at the output terminal with diodes to allow output but no return path. The sense wire is connected to the chassis batteries only, and if the alternator is not allowed to charge the chassis batteries by a bad cable, connection, or isolator, it has no way to charge at the correct voltage, and therefore, it overcharges trying to get the proper voltage on the sense wire. At bad sense wire will result in overcharging on both battery banks.
As for adjusting the output voltage, some but not all alternators have an adjustable regulator located on the top rear of the alternator. You just turn the slotted shaft to achieve the desired, but you need to fix the chassis battery charging problem first because the regulator can not regulate a charging voltage that it can not sense. If your alternator does not have an adjustable regulator, even Leece Neville has discontinued their production, there were aftermarket ones were still available the last time I checked.
Gerald