Very cool bus! Looks like a good bit of work has already been done for you.
You'll likely have an add-on electrical system in addition to the stock bus wiring which may or may not be a total mess depending on who had their hands on it previously. A catastrophic failure like yours is almost certainly going to be close to the batteries or in an electrical panel somewhere. A fried fusible link may show up as a burned spot on one of the battery cables or the cable may look perfectly normal. If the batteries are OK (12.1 to 12.7 volts) test the battery cables for continuity. If one tests as open, there's your problem. Of course you'll still have to figure out what in the jack circuit caused the failure.
Look at it as an opportunity to get way too familiar with your electrical system.
A battery at 10 volts is pretty much dead - 12.1 volts is about as low as you'll want them to go and then only after some heavy cranking.