I prefer to follow the generator manufacturer's recommendations when it comes to exercising it monthly. I like knowing it will start right up when needed without having to screw around with anything. When Onan says running it is needed to keep the slip rings clean and the windings dry, that's sufficient justification for me. As to the fuel tank, I want it full when the coach will sit for a time to minimize condensation buildup, and if it'll be sitting long enough I'll add Seafoam or now maybe the new CRC stabilizer that's formulated for ethanol laced fuel. That said, adding a small "test" tank for the generator, as the OP asked about, is not a bad idea, just not really necessary. Doing so would be a pretty simple project though.
