My grandfather had a sailboat, and I would help him winterize it...it had an inboard diesel motor, so we added anti-gel to the tank and ran the engine for about an hour. We totally drained the fresh water tank before adding RV antifreeze to it, and I believe we added some sort of anti-freeze to the septic tank as well although I don't remember specifics (it's been about a decade since he sold it
). Come spring we'd fill the water tank and run the sink until it ran clear for several minutes.
I'd imagine that an RV would be similar.