I agree they "should" be, but with such short runs usually found behind the typical RV panel, I really don't think its necessary. I have seen and run many single wire RS232 lines in RV's, never had a problem.
Probably not, unless your audio lines are unshielded. RS-232 isn't used for audio. What would have a greater potential to add noise is ground loops. I would refer you to Bob Nuckolls book for more information. He may also have a white paper on his web site.