Simon, the panel for the genset will have a switch to select either shorepower or genset,as the power source. Easy to forget to switch over.
Running a genset a long time with no load can glaze the cylinder bores, excess blue smoke is an indicator, 25 mins should not be long enough, and it should still run, give it a good hour test run, with load, like the water heater, an electric heater,fridge compressor, etc. The Onan book says you should run weekly for 1/2 hour.
If we ever get locked down again,Maritime`s site it was ok to visit the boat for checks and maintenance, and it wasn`t LGA limited. I kept a print of the page in the car in case we got questioned.
2005 Integrity 386 "Sojourn"