[JRTJH]

To determine what the roof decking is on your trailer, pull the trim molding off of the inside bathroom roof vent. There are 4 screws holding the "big plastic flashing" in place. Pull that down, you'll see the framing for the roof vent. There will be 4 diagonal "TPO flaps" where the factory cut the roof membrane to install the vent/fan. Pull the staples from one flap, you should be able to lift it and see exactly what's stapled to the top of the rafter. My guess is that you'll find 1/4" OSB.

As for securing the corner of the roof/sidewall flashing, I'd go ahead and secure that corner with a countersunk SS screw and an appropriately sized THIN fender washer (so the screw head is as flush as you can get it). Once you're satisfied that the flashing is secured to the roof decking, clean the entire surface with 409 or Fantastic cleaner, follow that with an alcohol wipedown, let that dry for a few minutes, then apply a 4" strip of EternaBond tape, centered on the screws. Roll that to properly secure it to the TPO membrane, then follow that with a "liberal 2" wide bead of DICOR self leveling sealant (1" overlapping the EternaBond and 1" overlapping the TPO. Apply the DICOR to all 4 edges of the EternaBond.

That should give you a "PERMANENT" repair. Don't even think about pulling it up again, so be sure your repair is "what you want" before applying the EternaBond. If you're concerned that the "rest of the flashing might move around and wear on the TPO membrane, I'd go ahead and secure it using the same SS countersunk screws and fender washers. If there's any concern with the flashing cutting the TPO at a later time, I'd fix it now since there's little/no chance you can pull up the Eternabond once it's sealed to the TPO and then sealed with the DICOR around the edges.

As for the front cap/roof seam. I don't like to use EternaBond there. It's too "permanent" and there's too much risk that a screw/staple will pull out and you'll need to access that aluminum molding strip. I prefer to secure all the screws on the molding strip, clean everything (as above) and then apply DICOR self leveling sealant to the entire cap/roof seam, completely covering the aluminum molding strip and screws and overlapping the front cap and TPO roofing at least 1" beyond the molding strip.