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.
Bleeder screws are usually found at the high points of a cooling/heating system.
On underseat heaters, look at the case. Most likely on the aisle side there should be a bleeder screw. It will look like a bolt with a screw head in the center. It will most likely be a 7/16" or 3/8". Loosen the bolt a little and then screw the center out until air and water start to come out. Screw it down tight again when no air continues to come out. It make take several cycles of letting air out before it is all released.
I have a bluebird tc2000 with a 12valve 5.9 cummins.
The individual heaters do not have a bleed screw. If you open the hood/grill on the front you will find a small 1/4" bleeder valve in a skinny tube tucked in above the radiator. You are supposed to remove the radiator cap, direct the tube back into the radiator. Then open the 3/4" heater supply valve at the top rear of the engine block. Set the engine rpm at high idle-1500rpm. Then open the 1/4" bleeder valve at the radiator. This bleeds the heater system. Refill the radiator if the level drops. It says not to run the engine longer than 50 seconds unless a flow of coolant is evident at the vent line. What confuses me is it says afterwards to open the heater return line. So maybe the heater return line was supposed to be closed when the heater supply valve was opened? If so they forgot to mention it in the procedure...
And that was for a front engine tc2000 bus. The procedure for the tc2000 rear engine is different.