The gauge swing when you turn on the ignition switch is normal.
Our high compression diesel engines require good batteries to start. The sound in the video sounds like low or bad batteries or perhaps a battery cable problem.
I would disconnect the batteries from the bus. Charge them thoroughly and try again.
I would recommend at least 24 hours on a good charger.
If that doesn't resolve the problem then take the batteries to the auto parts store and have them tested. If bad then replace.
If that doesn't work then start looking at battery and ground connections. They should all be CLEAN and secure.
If that doesn't work then ask someone smarter than me
There are plenty on this board who qualify