If you are getting no continuity from P1-20 to ground, either that wire is broken or the magneto is bad.
To check the belt drive, the genset must be removed from the coach to be able to remove the base cover. There is no way around it. The magneto is mounted under the flywheel, so it must also come out to replace the magneto.
You can test the windings without removing the set. To test the resistance of the field (rotor), look at the generator above the controller. You will see a small plastic 'plug' with one 10mm bolt and 2 wires. This is your brush holder. Simply remove the two wires, and ohm across the terminals the wires were attached to.
To test the main windings, remove the controller cover, disconnect the controller P1 plug, disconnect T1 and T3 from the circuit breaker, and T2 and T4 from the grounding post.
Main winding 1 is T1 to T2.
Main winding 2 is T3 to T4.
Quadrature winding is P1-3 to P1-9.
All of them should fall within the specifications in your manual, none should have continuity to each other or to ground.