Running a generator to exercise it when there's shore power?

If memory serves, Onan says to run under load for 1/2 an hour every month. It doesn't need to be a full load, but should be something noticeable. (As always, you ought to give it a minute or two to warm up before applying a substantial load, and let it cool down without much of a load before shutting it down.)

I don't generally manage that frequency of running it, but I do make an attempt run mine every now and again for a decent amount of time; it maybe averages out to every couple months.