[JRTJH]

Attached is a picture of the profile of the typical awning rail extrusion. If you install it as it is shown, any water running down the side of the trailer will meet a "smooth surface" and be shed off the extrusion onto the slide cover. If you install it "upside down" (as you have done) any water that runs down the side of the trailer will "pool" in the cavity at the round curve in the extrusion that is formed when it rests against the trailer fiberglass siding. That water, if it freezes and thaws during the winter will force the extrusion to separate from the fiberglass siding and will create a void filled only by the caulking.

Once the caulking becomes brittle, water that pools in that cavity will then be able to seep behind the extrusion and enter the fiberglass siding at the screw holes/rivet holes. Once that happens, water damage to the fiberglass siding is going to occur.

Could it be that the "Dometic expert" you talked to has never actually installed awning rails on fiberglass trailers and is simply "reading from a script of FAQ's" ???