Many times what a coach builder designs is not quite what you get when you drive it off the lot. As "stuff" is added, and it may be a newer modification of a system already present on the original design, additional needs for power need to be filled, so... another source is found, anywhere. Many times this is the reason for people claiming they shut off all coach power and then found dead batteries. Someone put a power tap at the first convenient post.
Depending on how accessible your mystery draw wire is at the alternator, I'd disconnect it for your trip and see if something obviously shows up. If that lead is hard to access, I'd extend that wire in a large loop so the loop is easy to get to. Then if you cut and tape the ends and you later find it powered something you need, a simple splice will but things back on line. You can always clean up the added spaghetti when you get to your destination or even back home. Remember too, the wire loom installed originally on your coach may also be used on other models and even if that lead was sitting there unconnected for a reason, used on a different model(?), some helpful soul may have connected it just because it was there.
2013 Tiffin Allegro Bus 43QGP
2011 Chevy Silverado 1500 Toad