Just like on all of us gravity takes its toll . Ive had campers that sagged like that in 3 years . look down the ribbed alum just above the cabover , from back to front , I bet the cabover has found a spot it likes down an inch or more and the alum siding wont go with it with out popping out or cracking . I doubt the cabover is loose , it just settled , you probably cant even notice is to bad from a profile , side view
If you really what to work at it you could pull the trim and the staples on the edges of the alum , reattach it in a more relaxed position and slid a small piece of alum behind the crack , with some caulking behind and pop rivet it together
A lot of newer campers and 5th wheels get stress cracks like that on the nose of the 5th overhang and around the corners of slideouts , and the fix is usually a small piece of flat white trim to cover it
But defiantly at least caulk it so you don't get a real big rot problem later
