[prairieschooner]

Bill,
Yes that is high at around 15 Volts. The articles I have seen for deep cycle batteries is 14.4 Volts for flooded lead acid. But the voltage regulator should be adjusting the voltage providing your batteries are in good condition. Float charge is down around 13.3 Volts.
You should most likely start with checking the batteries and cables to see if that corrects the problem. Overcharging could burn out the alternator because i would run hotter than designed.

The inside of my alternator was packed with gunk. Ours is flange mounted at the rear of the engine off of the camshaft and it looks like the seal was worn and allowed an oil leak. It wasn't dripping oil but allowing dirt to collect. This most likely kept the alternator from cooling and the rotor burnt out. The replacement rotor is around $170.00 alone providing the stator is still good this will run a little over $300.00 with me doing the R&R of the alternator.
Looks like it is just another item to keep my eyes on, if you see the gunk collecting near the base of this flange mount, it is most likely time to replace the seal. I will be calling a Detroit Diesel parts supplier later to try an get parts.