If it were me, I would replace all four of them, ( and service the brakes and bearings ) not only for the peace of mind for the upcoming trip, but also as a selling point. Polish up your salesmanship for the upcoming sale, and be able to point out to buyers that ST tires require replacement after three or so years, and that brakes and bearings need regular PM also. Let your new tires and service be a up-sell point for the trailer. You should be able to get the bulk of your new tire/service investment back out of it.