In what world is the maintenance cost for a fleet of EV busses going to be less than that of a diesel? There are two orders of magnitude more diesel mechanics than people qualified to work on EV drivetrains.
It's a pretty big transportation sector we're talking about (half a million of these buses in the US), so it will not be a fast transition and early adopters will be paying higher costs than later ones to iron out the kinks. Most districts are probably waiting for someone else to try it out first.
Fewer fluids required, just your differential and some coolant for the motors really.
Given the specialization of parts, it's far more likely that any performed maintenance is going to be more of a swap for known-good parts and sending them off to the home-office for reconditioning.