How do I remove decal residue

I had one of those static cling decals on the back of my Jayco Eagle trailer. It was on for several years. The decal became discolorded and the transparent edges became brown. It took a razorblade to remove the static cling decal. Once removed the very edge of the cling left a brown residue outline on the trailer where the decal was, that the razerblade cannot remove.
I have tried Goo be Gone grafitti remover with no luck. Any suggestions on how to remove this brown residue from the fiberglass?