I once took delivery of a motorhome and set out on a 4,000 mile trip the next day, and experienced a similar problem. Since it was a case of "load it and drive it", we didn't really have time to check everything over. Within a few miles, however, I realized I had an overcharging problem , in that the voltage needle buried itself at the top of the gauge when the engine was running (18+ volts!!).
So I'd disconnect the alternator wire and run it on battery for a while and then hook it up and charge for a few minutes, and then repeat til I got to a dealer (this was a Ford Class A). Told them I needed to get back on the road as soon as possible, and they fit me in as best as they could (less than 2 hours, pretty amazing!). While I was waiting, I looked at the coach batteries and then did a double-take... THEY WERE BONE-DRY!!
Evidently, the alternator saw a huge load due to the dry-batteries when underway and just kept pumping the juice to everything, including the coach and engine batteries, jumping the voltage waaaaaay up there. Put some water in 'em and it worked fine after that. I gotta admit I also let Ford buy me a new alternator, just to be sure.
Had another instance in the mid-80's where the 'sense' wire to the regulator came off the alternator (I think the regulator is now built-in to most/all alternators) in a storm and caused the voltage to skyrocket. Will never forget the sight of the wiperarms going so fast they threw one of thewipres right off the arm and onto a vehicle next to me.
Not sure if either of these apply, but food for thought. Good luck.
- - - - - - - - - - -
'06 HR Endeavor 40PAQ (Mission Hills decor). All options, but still finding more to add.