Always use the big lock, unless the trailer latch can only fit the bolt or cable seal on the right side door, making it impossible to use, if that is the only place to attach said seal or lock. And try to learn what places have higher risk of b.s. like this happening than others, and plan accordingly.
all jokes aside. you should be doing a walk around and tug test everytime u get back in truck. i set trl brakes then pull on trl a little and set truck brakes. cant pull fifth wheel latch with pressure on it.