About five years ago, I covered the side seams of my fifth wheel with 4" wide Eternabond, all the way down to the top of the drip rail and right on top of the vinyl insert and cleaned caulking; there is about 1 inch of Eternabond stuck to the roofing material, then an inch or so stuck to the caulking, with the remainder attached to the railing. It is still stuck firmly today.
You probably already know this, but:
Clean the drip rail and roof area thoroughly; all chalkiness must be removed as well as all dirt. First I cleaned the area with a liquid cleaner and a brush and rinsed well and let it dry. I then used naptha, sparingly and carefully and it worked well. Others say to use acetone or Eternabond's branded cleaner.
Eternabond is the stickiest stuff ever. Apply it carefully. I tried to align the bottom edge of the Eternabond where I wanted it to end up on the drip rail, and then folded the remainder - evenly, carefully, and a little at a time - over the edge of the roof.
Roll all the Eternabond firmly down.
