TT Wear on one tire..only took few short trips

We have a 21 ft TT barely used. We prepared our truck and TT for long awaited trip out west. When we arrived at South Dakota we noticed three tires still looked new but the fourth looks like it has 50K miled on it. It shows a lot of wear. We are very concerned but have no answers as to what it could be. Would appreciate any ideas. Thank you. Larry