This is the corner molding that goes over the metal sides on the outside 4 corners?
Lay tape on both sides of the molding itself leaving an excess, maybe 1/4" or so, hanging loose. Don't install the tape flush with the edge of the molding. Prior to installing, place short pieces of tape in the low spots of the siding so when the molding is installed, you'll have a double thickness there. Push the molding into place starting on the lower straight sections. Using a soft rubber mallet, tap the molding into place (easy does it!) to kind of mash the molding, tape, and siding all together - just the straight section! Once it's kind of seated into place, run a screw in just until it's snug. Too much at this point will deform the molding! Once all the screws are in and snugged, you can go back over them by hand tightening a little at a time until the tape starts oozing out from under the molding.
With the straight section of the molding pretty well fastened, you can work into the curve much easier. There's a little finesse involved, so take it slow working maybe a foot or so of the molding into place at a time using the mallet and screwing just that section down, then moving to the next. By the time you get done with the trailer, you'll be getting pretty good with it.
When the molding is installed and final tightening is done, run a flat bladed screwdriver down each side of the molding to trim the caulk neatly. This will expose any voids in the tape, which you can then pack with the excess you're removing using a putty knife or the screwdriver.
We used 1 1/2" screws so the wouldn't strip too easily while drawing the molding down. Shorter don't work as well... If the wood is soft, it's not even worth trying. You'll have to replace it as it takes pretty solid wood for this to work.
I don't get in here often, so if you need more help, shoot me a PM. Be glad to help. Used to do this professionally (20 years).
1997 37' HR Endeavor, 275hp Cat, Freightliner
03 CR-V Blue Ox, Ready Brake