Ideally you should run an engine long enough with enough load to dry water condensate out of the exhaust system and crankcase. I try to do that on all my engines. If you can try to combine generator jobs toward that end.
It is not always possible. Hopefully the Onan isn't such a delicate flower that it is injured by occasional lapses. The one thing I will do is run it under load for half hour plus prior to putting the unit away for a length of time. I don't want the condensate sitting in the crankcase or exhaust for extended length of time.