Hi neu,
This pic of yours,
I know you are asking how to repair it, but ideally, when a rip in the membrane happens, it raises the question, what caused the membrane to rip?
What year, make, and model is the camper? Did you buy it new, or used?
If you can post more pictures of that entire area ( we need to see a foot or more on the roof, the left wall, and the front wall, that would be helpful. Also, post the other side of the camper so we can see if this manufacturer does the same placement of the corner molding the same on both sides.
The black corner molding in your pic "might" not be in the right place, it "appears" way too high on the roof. The corner molding is supposed to be flatter on top of the roof; the molding side with the screws in it holds it down to the roof. I can also see what might be the top of the gutter rail in your pic, the corner molding is normally closer to the top of the gutter rail, and in this case, it is not. Something may have been assembled wrong or something hit that corner molding and it is lifted up off the roof.
The four corner joints of a camper have a lot of flexing stress going on, and as the camper ages, sealant leaks start in the roof-to-wall four corners due to the flexing. While just caulking may stop a leak now, if something else is going on, Dicor caulk will split faster than normal from the long-term flexing if something tore the membrane.
There is not enough info from your one pic to tell you how to make a repair that will last and not come back, as the cause is not yet understood.
Hope this helps
John
