Another quick way to determine if the cause is the alternator or regulator is to remove the field wire at the regulator and touch it to a 12 volt source while having the positive lead of your meter hooked to the alternator. As soon as you touch 12v to the field wire, the alternator immediately ramps up voltage, confirming the alternator is good and the regulator is bad. Don't hold the field wire to 12v for very long, it just takes a quick touch and a spark is normal.
Just had to do that this week on the Prevost. The regulator for the D50 alternator was 87.00. Don't forget the jumper plate when changing out the regulator.
