Most diesels have a solonoid in the engine compartment that the start portion of the switch activates. Find this solonoid and test the small wire on it to see if it gets the signal from the switch to start. If it is not getting power, you have a ignition switch problem. If it is getting power, you have a solonoid problem.
1998 Pace Arrow 35 ft. F53 Ford V10 2012 Honda Civic toad
15 year motorhome service manager. 3 popups....2 travel trailers....5 motorhomes....loved them all.