I am going to start by stating the obvious--make sure the bus is in neutral. I can't tell you how many times I have received calls that a bus won't start and I have asked if the bus was in neutral and then the bus started right up.
On a Crown, regardless of whether or not it is a manual or automatic, there is a neutral safety switch. Both do tend to fail over time.
If when you turn on the key and buzzers start buzzing and the transmission is in neutral and the starter still doesn't turn you will need to start going downstream to discover where the juice stops.
But before you start checking circuits the very first place to look is to check the engine control panel on the street side engine compartment. If the start/run switch is in the wrong position you will have no juice at the ignition switch. While you are there see if the starter will engage with that switch. If it does you will know the neutral safety switches are good, the starter solenoids are good, and the problem is either in the start/run switch or the ignition switch.
The next place to look is at the first solenoid in the system. It is a solenoid that is always hot when the key is on. It powers everything up. If your buzzers are not buzzing this would be a good culprit to suspect. It is located in the electrical panel in the step well below the front heater unit.
The next place down the line is the solenoid inside the curbside engine compartment. That solenoid carries the signal from the switch to the starter solenoid.
The battery cables start at the batteries on the street side and go across the bus to the battery shut off switch in the curbside engine compartment. After the master disconnect switch there should be some very large fuses that protect the main leads. From there the main leads go to the (+) side of the starter solenoid. There are also other leads that go to the alternator and main electrical panel in the front of the bus.
The weak link in the system is the solenoids in the front electrical panel. When they were made by Cole Hersee or AC Delco in the US they lasted a long time. Replacement solenoids tend to be made in China with very variable quality control.
Good luck and if you have any specific questions feel free to contact me.