I believe that I experienced the same problem. When you turn the key to the on, not start position are you getting any indications of the wait to start or warming up Glow plugs.
I have a 2002 Thomas and when this happened to me it drove me crazy to the point of scrapping the bus and curling up in a corner and pouting.
Inside the Electrical compartment under the Drivers window (My Bus) don't know where yours is. There are two Solenoids, one is called "Body" and the other is called "Chassis".
The purpose of those are when the key is in the on position Voltage is transferred thru them to the various parts of the vehicle. Body solenoid is for all the accessories and lights, and the Chassis is for all things related to starting the Bus i.e. ECU, Glow plugs, Intake heater and initial ECU Diagnostics.
Check to see if when your key is in the on position that you have voltage between the two large posts on the solenoids, if not check to see if you have 12 volts to the center post when key is in on position. If you have 12 volts to the center post and nothing on the outlet of the solenoid then you have a bad solenoid. Also you should have continuous 12 volts to the inlet of the solenoid and if not trace down stream and check for fuses.
Here is a picture of what I hade to replace. Amazon $55.00 bucks.
White Rogers 100 Amp Continuous Duty 3 post Solenoid