Eric, a tip given to me by the Reese representative: get a bottle of red/red nail polish, and paint a line from your hitch plate down across the nut. If the nut loosens the line will, of course, be "out of line". Tighten it once and paint another line (unless the first one lines up). If it gets loose again, replace it!
(This assumes that the lock washer is between the nut and the plate; i.e., next to the nut, and that it was initially installed very tightly.)

Donald WBCCI #2875