Always use grease, clean it off once in while and re grease.

I remember reading somewhere that if you don't grease it the two can gall together enough to cause control problems. Mine squeaks when it needs grease and I clean if off and re grease, both the ball and the hitch.
Can't imagine trailing across America at 75 mph with A DRY HITCH, THAT'S ASKING FOR TROUBLE,IMO. I have made a bunch of trips out west and go 5000+ miles before unhitching my trailer, it can't go that long metal to metal without wearing out and possibly coming off the ball!!

I've done a bunch of trailing, your nut's if your on our highways with a dry hitch and a danger to the rest of us.