[Richard]

The chopped off and capped wires are extras that Newell typically adds.

On that era Newell you will not have a battery isolator. Newell uses a merge solenoid, the cannister in the lower left with the big cables attached, to connect the house and chassis batteries. Here is where it gets tricky. I have seen them wired two different ways from the factory. One way requires the use of the merge switch on the dash to activate the solenoid. The second way is that the merge solenoid is energized when the key is turned on.

So as a first order sanity check, identify which of the two cables on the merge solenoid is the chassis side. Check continuity between the end of the cable attached to the solenoid and the end attached to the alternator. If it has continuity look no further. Now, do the same for the Neg cable at the alternator to a known good ground.

I know you said the alt was wired as self ignite. I would check with the alternator shop who tested it to make sure that was the way they tested it. They could have applied a 12V source to the ignition post. That's a long shot, just trying to imagine all the possibilities that explain what you are seeing. After all, you said it has not worked since you owned it.