I'd try this: Coat the mechanism with penetrating oil then scrape off as much rust as you can from the jaws and any other moving parts. Always be sure you lube the ball before use. Any lube will be better than nothing but I use a Reese ball grease which is simply a "white lithium' grease. It can be purchased at most any auto parts store and many other places. After the season is over OR after driving through a bery dusty/dirty area clean the coupler and ball with a degrease agent but wouldn't bother until it's working smoothly again.
