If it were me, and I was convinced that there was no stud to hit, I would drill all the way through the wall and install a wood or metal backing plate on the inside of the trailer. I would drill through that backing plate and run bolts all the way through. I would fill the bolt holes with silicone caulk, and I would put a square of butyl tape at the outside of the hole, between the rail and the side of the camper.
It's pretty radical, but it would give you the necessary support for your rail. There's not much that's worse than a safety feature that is not safe.
I would have to be completely convinced that there was no stud available before I would do this. That would be a difficult thing to convince me of.
-Speak
It's just Mrs. SpeakEasy and me now (empty-nesters). But we can choose from among 7 grandchildren to drag along with us!
2014 F-150 Super Crew Short Bed 3.5L Ecoboost
2014 Flagstaff Micro Lite 23LB