Everything works on Shore Power, Nothing on Generator

You probably have an answer from your repair shop by now, but I will weigh in anyway.....free advice. Being a Guardian will change symptoms/causes as it is diagnosed. Guardians will actually continue to run when they are not producing power, where Onan will shut down if no ac power being generated. Voltage regulator most common problem on these sets, but need to check rotor field brushes as high resistance here will cause short regulator life.