If it's starting easily and able to run the roof AC and microwave oven at the same time, it's performing well. Just start it once per month, let it warm up and then power on the AC and microwave and let it run for an hour. Shut off AC and microwave and let it cool down for a while before shutting generator off.
Change oil per Onan recommendations.
It's important to keep the electrolyte levels covering the plates of the house batteries and check that they are being fully charged to 13.6 volts by the converter charger when connected to "shore power" or by truck alternator. If kept charged, not run down too far, batteries can last 4 years plus and start the RV generator reliably.