I had a very similar issue with my Chevy Express. Several years ago I had a HD alternator installed,about a week later while going to work I felt the engine load down a little, checked the gauges and the voltmeter was at 1/4 instead of the halfway mark @14v. Took the van back to the shop and they replaced it and said it must have been defective. Was fine for about 3 years. This past fall we planned a trip to Colorado, Wyoming and South Dakota from Florida. When prepping the van for the long journey I basically replaced everything on the front end, water pump, power steering pump, belt, all the pulleys and a Bosch HD alternator and start battery. On the return trip after staying at a campground where we plugged into shore power, so the 2 group 31 AGM house batteries had a full charge for the next day and eliminating a heavy load on the alternator. After starting out before dawn fired the van up and after a minute my light for my BlueSeas ACR came on so the house batteries were connected to the alt. About 4-5 hours into the ride while checking my gauges my voltmeter was on around 10v and dropping fast and the batt light was on. Told the wife we are about to go down and to get on the net and find the nearest shop. She looked it up and there was a Chevy dealership at the exit we just passed. Knowing we only had a few minutes left and we would not make it to the next exit I banged a u turn at a emergency only spot in the guard rail, got off the exit and started down a country road and crossed our fingers trying to make it to town, watching the meter drop I told my wife we are not going to make it after a couple minutes we sputtered to a stop in the middle of nowhere in Missouri. To our luck there was a farmer at a building where we pulled off. He called a tow company and suggested we not use the company that he was going to recommend because they were crooks and get towed to the dealership. The dealership was a three ring circus, when I explaned the ACR, I got the deer in the headlight look from the tech, he went to the Mgr and said he didnt understand what a ACR system was, Went out to the van and it started! The tech checked it and said he couldnt find a problem, I looked at the Mgr and said there is a problem and that the voltmeter was reading low and I want the alternator replaced and he agreed. Next issue, they didnt have the alternator in stock so the sent a driver to ORiley Auto Parts down the street which took about 2 hours! and doubled the price. The first thing I did when the problem developed was disconnect the ACR and the house batteries and checked the 75A fuse from the alt to the ACR and the 75A fuse at the end of the circuit back at the house batteries which were both good, eliminating the house system. The new alternator has been no issue for several trips but it always in the back of my mind and never take my eyes off the voltmeter. Im interested in you post to see if anyone can come up with a explanation of this issue. Any other super long trips I will probably take a backup alternator. When I look back it could have been a lot worse having happened in the backcountry and not on an interstate.