It all sort of depends upon how many heaters/defrosters and feet of coolant lines you have in order to determine how much coolant you will need.
Just the engine and radiator is going to take about 5-gallons. Add a few heaters/defrosters and 100' of coolant lines and you can be up in the 20-25 gallon range.
As far as draining the system, there will be a drain on the bottom of the radiator. In addition to those there should be drains on the bottom of the water jacket on the engine block.
As far as getting rid of air in the system, all of the heat exchangers in all of the heaters/defrosters have a bleed screw. Fill the radiator just like you would your car or pickup. Once the radiator won't take any more you can then start bleeding the heaters/defrosters. Don't be surprised if you have to bleed the system the next couple of times you run the bus.