The closet is not difficult to install but as you identified the closet is a structural item. The closet edge next to the doorway is fiberglassed to the outer shell, when you fiberglass
this in make sure you support the roof and check the door fit. At this point you can adjust the sag in the body and improve the fit of the door.
The remaining edges of the closet (top and window edge) are pop riveted to the body
Hope this helps and good luck finding a closet.