I have the service manual for our Onan 10 kW HDKCA generator. It does not say where the solenoid is located, but it suggests that it is probably visible once the top cover of the generator unit is removed. I am expecting the solenoid to be a part of the starter, similar to most automotive starters. The manual says there are three relays, located in a row above what they call the "intake resonator". It appears that K2 is the starter pilot relay. I hope that helps.
Jim & Angie, Boise, ID
2006 Monaco Dynasty Countess III
2017 GMC Acadia Denali