Motorhome Generator

I have the Kohler Model 7CM21 Generator 7,000 watt in my 1984 Motorhome. I am having issues with low voltage and incorrect hz. The Generator runs fine no surging output is 110 volts with no load and 105 volts with 1500 watt load. The main issue is the hz, at no load it is 275 hz and with a 1500 watt load it is 273 hz. When I turn off the Generator and plug the Motorhome electrical cord into regular AC outlet from the Garage I get 121 volts at 60 hz, in the same outlet in the motorhome that I measured the Generator voltage, so I know my meter is accurate. What are some of the items I should be checking to improve my output voltage and obtain the correct hz?