The best i've found is called prep solve use by body shops before priming it's also great for removing old decals & glue if you let it soak into them before scraping
Ive used a couple different forms of gunk/gum remover. Ive used stuff like goo-b-gone thats in a tiny metal can. Ill have to get the name of it later, im not at home, but it worked great.
The best thing to do is use the products that your decal makers use when they do cars. I always found it worked the best and wouldn't dull the paint finish.
Oh, also the best thing for black streaks on your enclosed trailers is a product named purple power. The only place I could find it is Pep-Boys. I used it on the greasy parts of the car to and it would work great.