I believe that the back end cap is a one piece down to just below the brake light assemblies so it would be detached up on the roof and probably a few places on the vertical section after removing all the marker and brake light assemblies, than lifted up like a door so you can access things underneath. That's one way anyway.
Not sure where you've spotted the nest, but here are a couple pictures that may help:
And though mine is a Journey, this picture is of an Itasca:
These days you can buy an inspection camera with a 6 foot probe to get up in there and view where the nest is for under $20. Works with Blue Tooth displaying on your phone or tablet.