In addition to what Bill wrote, the shells can also be in the same plane, and still be out of alignment. If the two halves open or bind when the door is closed, the shell is not parallel with the box. See post 17.
https://www.trailmanorowners.com/for...t=shell&page=2
This will cause the end of the shell (front end for the front shell, rear end for the rear shell) to sag slightly. On the front and sags, the top door will separate slightly from the lower door when closed. The solution here is to adjust the shell height by increasing it by a 1/4 or 1/8" to compensate. If you raise the shell too much, the door will bind.
The specification for shell height is measured FROM the bottom of the aluminum skin on the box, TO the bottom edge of the lift arm bracket inside the pocket stop when the shell is locked in the open position. The specification is 39.5", but if you have the shell sag, you could increase this to 39 3/4".