I would first start with checking things that drive the alternator, like belts, as a slipping belt can let it turn too slow. But then there are always times when the alternator just fails and something as simple as bad bearings can leave it running too slow or wiring can be beginning to go bad.
I ight first disconnect as much of the electrical from the alternator and check how the output looks then, before changing the alternator. Also it would be worthwhile to check the voltage with a meter so that one is not trusting the onboard stuff to be good. May be a bad alternator output but it might also be a problem with reading that output.
Just pays to check before spending time and money to cure the wrong thing.
2015 Winnebago Vista 31KE on 2014 chassis