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If your Thomas is anything like mine, a 40ft RE, you will have three “main” harnesses.
1. Body wiring
2. Engine wiring
3. School wiring
This makes it a bit easier to sort everything out.
After years of monkeying around by well intentioned School district maintenance, they can get a bit jumbled.
You will absolutely need a digital voltmeter.
If you can go on Google and look at vehicle wiring harness from the era of your bus, you’ll start to see how things are typically wired.
School bus wiring is a bit different with all the interlocks, alarms, flashers, etc... but still straight forward enough to sort out.
The body harness and engine harness are extremely similar in function and layout to GM stuff from back in the day.
Seeing as this is a GM chassis, this shouldn’t be too hard at all.
Best way to do this without a schematic is to carefully trace each wire, visually as much as possible, and then confirm with the DMM.
Thomas had not been all that great about making stuff available to the used market. I was lucky and the main body harness was printed out and put on the electrical access door. Still had to figure everything else out, but I have a couple of decades of messing with GM wiring.
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...but I think every bus owner should have a Power Probe. It would absolutely help you figure out what's what in that mess of wiring. It's essentially the digital voltmeter Klingon mentioned, but with handy functionality that is super helpful. 
Linear Mode
