Go to the auto parts store and get a starter jumper switch Hook it to the starter and hit the switch. If it starts that way then the starter isn't the problem. Could be the neutral switch or the rely in the rear left fuse panel. Check the two clear ice cube rely fuses there. You can pull the heater one from up front side panel and exchange it for the one back there to see if it starts and if it does then that is all it is. You can buy a new one at any electrical supply. Neutral switch is at the right hand side of the tranny by the shift linkage and can be jumped with a paper clip to test.
Clifford you taught me that LOL with the Paper clip. You should have two wires about where your linkage shaft is on the right hand side of your tranny. Put the paper clip into both wires and then try to start it. If it starts it is the switch because you just by passed it. If not then remove the clip and try the starter switch button on the starter solenoid .
Have you run a multi meter on the pos and neg going to the starter to see if you have enough power if any?It sounds like you don't have the sensor on yours probably only on older buses as is the flapper in the air intake.Your profile says series 60 and the above says 6V72 so I would think you mean either 6V92 or 6V71Just thought I would ask is the shifter in Neutral when you are trying to start it?Start looking for grounds from the starter and take them off and clean and replace them both ends. Then try again.