As Dutch pointed out and everyone else ignored the main point of running an Onan per the manufacturer's recommendation it to keep the electrical parts in good shape. Sea foam, draining the carb, etc. have absolutely nothing to do with moisture and corrosion in the slip rings. They have nothing to do with moisture and corrosion in the windings.
OP,
I always used a little electric heater to load my 2,500 Onan. The 1,500 watt heater was a bit above 50% load. If you have a 4,000 watt generator a 1,500 watt heater is still probably fine. A larger generator may need two heaters to get approximately 50% load.
Is this RV parked near your house? Run a cord into your house and run the heater in there. This will help keep your house warm for you.