I have been around a few MCI's of that vintage and older. I have NOT worked on them myself so I am simply repeating what I have heard. Take it for what its worth....
As best I understand it earlier MCI's had 24v coach batteries (12v in series) and ran most of the coach functions, starter, headlights etc on 24v.
Some time later they switched most of the functions, lights etc, to 12 v using a 24v-12v converter but the starter remained 24v.
Some time during the MC-8 production they went all 12v.
I do love the Skoolie board and the people here. However, you will find more MCI specific knowledge on BNO BBS - BNO's Bulletin Board System - Index
or Bus Conversion Magazine Bulletin Board - Index
I will warn you that folks there are not the easy going bunch that we enjoy here........
I will third (or fourth) the advice given above. Get a volt meter and start checking batteries and accessories to see what is 12v or 24v.
Where are you currently located? If you are anywhere close to me I would be happy to spend a day with you and see if we can get your questions answered.