[Richard]

Hmmmmm, I cannot find a picture of that particular Leece Neville alternator. Rather than take a chance on a miswire and frying the alternator, I would go back to the alternator shop that checked it out, and ask them for a simple schematic of how it is wired. I would also find out if the alternator requires a regulator or it’s an internally regulated model.

Then you can start to diagnose.

Some quick things you can do in the meantime. The large red cable on the right of the picture appears to go to ground. You can verify that with an ohm meter. Like Chris indicates, that post is USUALLY hot on other Leece Neville models. This may be a problem.

The small red wire should have 12V on it when the key is ON.

And you will have to trace the heavy red wire on the left into the coach to see how it eventually connects with your chassis batteries.

It is always a good practice to take the time to completely understand how YOUR coach is wired. Many times, previous owners have modified what was original. The potential for letting the smoke out of expensive electrical components is high by using wiring examples from another Newell WITHOUT confirming that the wiring example is consistent with your coach.