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

I always ran my generator at least 30 minutes, but usually an hour each month. Actually I would take the motorhome out for a Sunday drive, once a month, about 50 miles each way. Run the genny with a/c load on it. Gave the tires/slides/jacks all a chance to move. And, always enjoyed the ride and the lunch in it. Was told not to let the gas varnish in the generator or I would have big problems.