The type of travel trailer lock you pick will depend on whether or not the trailer is attached to the tow vehicle or not. When left unattended, the best trailer coupler locks are those that both block the coupler and lock the latch pin in place. You can accomplish this with two separate locks or one combined unit. When attached to your truck, the best locks secure the trailer hitch into the receiver with a locking pin. Additionally, they secure the coupler latch closed so you cannot lift off the trailer the ball.