attach sunscreen fixtures to the outside of trailer

Make certain you're screwing into the frame and not just the siding.

Do you have an awning? Are you looking to put something on the off door side?

Awnings are attached by sliding the edge into a groove mounted on the trailer. You could pretty much do the same and slide something in and out at set up and tear down.

Carefully with screws and lots of Dicor? Post a couple of pictures please.