Darrell,

Follow the electricity. You have ruled out the starter solenoid, so using an ohmeter or continuity tester check the relays relating to the starting circuit in the electric box.

Also, as Dan says check the rear start selector switch. It may be passing current when in the rear start position, but not in the front start position. The key switch is another set of contacts to check.

You can access the electric diagrams for your specific model year chassis on the Prevost Car site. If you follow the starting circuit by working backwards from the starter solenoid you will find somewhere along the way you either have a switch with bad contacts not passing current, or a solenoid or relay either not functioning or with bad contacts. With the wiring diagram or the listing on the labels that tell the funtion of relays you can focus on just the components in the starter circuit. I cannot remember if you have a relay or solenoid in the front electrical compartment.