Some, perhaps all, towbars which swing down are stowed at such an angle that use of a towball cap will just collect water from road spray - "got the T-shirt" so don't use a cap on my VW Touareg.If you are not going to use it I would use ACF 50 anti corrosion spray, or waxoyl to coat it, but before using it again you would have to thoroughly clean it with a degreaser. Make sure you have two towball caps and one should never be fitted to the towball when it has any anti corrosion substance on it.
Our Volvo has the electronically-deployed, underslung towball. We just put the cap on it and put it away, then give it a light cleaning before next use. The longest duration without use is about 5 months and over that time it picks up a dull tarnish but nothing you'd call 'rust' or serious corrosion.If it’s only a matter of preventing rust while the towbar is unused and in the retracted position for a few months there shouldn’t be so much rust forming in that period of time that would necessitate any for of protection. Any rust film that may form during that time will still be so thin and loose that one can just wipe it off with a dry cloth. That’s all I do.