Sta-bil OR Seafoam??

Unless you actually drive the motorhome for at least 20 miles, it is probably best to not start it. The engine oil won't get up to full operating temperature (even thought the coolant temp gauge says it is) during 30 minutes of idling with no load. The purpose of running an engine is to get the oil up to at least 200 degrees F., so water and other contaminants will boil off. The (air cooled) generator, at approximately 50% load (1500 to 2000 amps for a 4KW generator) will do that in a half hour run, no problem. This also removes any moisture from the windings. An hour run would be better.