To fix a motorhome that won't start, check for a drained battery, dirt buildup in the cable connection points, loose or damaged battery cables, a defective solenoid and a defective starter assembly. These are the factors that commonly cause a motorhome's starting system to malfunction.
A motorhome's starting system is an electrical system, so any troubleshooting step must begin with the batteries. Clean any dirt build-up on the contact points, then check the battery's voltage and level of charge with a voltmeter. Make sure the voltage is at 12.4 volts or greater and the charge is at least 75 percent.
If the battery is fine and the battery terminals are clean and well-connected, the problem could be in the battery cables, the starter solenoid or the starter motor itself. To locate the source of the problem, use the voltmeter to check each connection, starting with the battery cables, then the solenoid, then the starter motor. If one or both cables show low voltage, that may indicate a loose connection somewhere in the length of the cables. Repair or replace the cables as necessary.
If the cables and their connections are fine but there is no voltage from the solenoid terminal to the ground when the solenoid is active, then you may need to repair or replace the solenoid.
If voltage is normal at the solenoid terminal, then the problem is in the starter motor or its wires. Repair or replace the starter motor as needed.