sorry to hear of the problem you have. I had the same problem for 2 years with techs telling my coach wasn't wired to charge except from generator or shore power. One even said the fuse slot was empty but in reality, he was looking at the wrong slot.
One day I was working under the dash on a different issue when I turned on the ignition I looked at a fuse panel located there. I noticed a red led light next to a 5 amp fuse. Hmmm, that is usually a sign of a bad fuse. I replaced the fuse just because it was blown and my coach batteries now charge from the alternator. Such a simple fix that no one seemed like they could find it.
I suppose it could be something simple or more complex but look for the most simple first.
__________________
|