Step up on the bumper and gently bounce up and down while watching that skin-to-bumper gap. Ideally, everything moves together. If you are having a bad day, the bumper will move up and down independent of the shell. This is know as (The Dreaded) Rear-end Separation.
Rear-end separation is caused by water getting into the rear of the subfloor, rotting it out, and creating room for the frame and shell to move independent of one another. Worn out axles, too stiff of weight distribution, unbalanced wheels, etc., then all work together to complete the job.
If your trailer hasn't already had the subfloor replaced, then it is time.