I have been having the exact same problem on my 2005 Fleetwood Discovery 35M, Cat 330 in a Freightliner Chassis.
Started shortly after July 4th in fairly hot (90's) temps.
First, I noticed a warning light on the dash, and the info center said the same thing as yours (warning "Auto Idle". And then underneath the warning was the system voltage displayed as 13.8V and then Threshold at 13.4V.).
You noted that you moved the coach and it stopped, I bumped the brake trying to read the info center, that is located low on the dash and hard to read, and the idle dropped to normal and the light went out. All seemed OK at this point.
Tap the brake which will drop the idle speed and the auto idle and too much voltage should stop.
I have had my coach for a year and this just suddenly started happing each time I started the coach after I had been boon docking for a few days. It did it 3 times with no other side effects, it seemed fine.
Then, I was driving on hwy (395) and a light came on "Low Voltage" 11.8V. I looked at my Volt Gauge on the dash and sure enough it was showing discharge. Slightly Left of Center. I was right in the middle of a big climb and there was no where to pull over. So I just reached over and started the Generator and decided to wait until I got to Bridgeport, CA to pull over and look at it. Within 3 to 5 minutes the meter on the dash began to show charge, needle slightly to the right of center. ( no number on my gauge just marks.)
Pulled over and shut everything off. Got out my multi meter and the battery showed 12.6V with nothing running and only minor load.
Started the engine and retested, 12.1V to 12.2V with engine running.
Started the Gen and initially it showed 21.1V but, as has been pointed out to me on another Discovery Owners Forum by Gary Osburn, The alternator charges the Chassis first and then moves to the house, and the gen charges the house first and moves to the chassis. Sure enough, since the house bats were full, within a few minutes the current at the batteries hit 13.4.
Not to drag this out to long, it was Sunday, in a small town and NAPA had just closed for the weekend… I pressed on using the Generator. I drove it into Carson City and parked in a CG with Full HUs.
Put it on the DOAI Forum and it seemed clear that all agreed it was a bad alternator. Called Cat, they wanted $750 just for the part. Hung up and called Freightliner, they wanted $450. Just happened that the lady next to me in the RV park had a Brother in the auto parts business and he sold me one for $350. Had it shipped to my sisters to pick up.
Picked it up and tossed it in a bin and drove to Colorado to a shop to have it put on, as I had to get there for other commitments and it was to HOT in Clear Lake CA to do it myself in my sisters front yard.
It was installed and we initially tested the batteries and they showed 13.4 … Success or so I thought. Paid the bill and off I went… Oh wait the gauge shows discharge again… U turn back to shop… 12.2 T THE BATTERIES AGAIN!!!
Having spent 3 days in the shop getting a bunch of stuff done and no showers plus hot hot hot, decided to go to my campground and think about it.
Then this morning, your post!
Let's keep each other up to date on this.
I did some more research this morning and decided I need to do more testing.
Pull the floors and check the output on the back of the alternator.
If it is producing 13V plus check the continuity to the battery to see if the charge is getting to the batteries throughout the wire.
Could be a bad/shorted wire or a blown fuse or fusible link.
Then do some more thinking and discussing.
I hope it is not the issues Rich has had. Ouch!
05 Fleetwood Discovery 35M
Cat 330 in a Freightliner chassis.