When starting the vehicle you just hear a continuous clicking sound. Have already removed starter and had it tested. It works fine at the NAPA automotive parts store. The vehicle is the chassis portion of a Tioga motorhome. Tested the voltage at the starter when installed in vehicle and it checks out good. Battery is putting out 15 volts. Have replaced the starter solenoid but that didn't make any difference. The weird thing is that there is also a starter solenoid for the generator and when accidentally touching both leads with a wrench the starter turned the engine over. So we replaced the generator starter solenoid too. Any ideas on what to check next?

Clean battery cable and starter motor connections with abrasive cloth. Examine the battery cables carefully for internal damage. The link below may help:

https://www.askmehelpdesk.com/cars-t...tml#post235560

You may have a bad ignition switch.