I put a bike rack many years ago and it had to be anchored to the side rails and not the middle, There was nothing in between except where the roof AC was mounted. Otherwise an aluminum skin, foam board and interior ceiling as far as I could tell.
2017 SLX 195RB
2014 Jeep Grand Cherokee Overland, gas 3.7 V6
Andersen WDH hitch
Dometic Milenco Aero mirrors