If you know you are going to polish and clearcoat sooner or later, you can be more aggressive in your removal methods.
Try bug and tar remover, from any auto parts store. It is a solvent-based cleaner, and really good for pine sap, but it might affect clearcoat, so if you have clearcoat you care about, pay heed to the "test inconspicuous area first" caveat! Another thing, apply the cleaner to a rag and wipe locally. You don't want it running into seams; it will attack the sealer!
I bought my streamline from the storage yard where it had been sitting for years. There was literally a half-inch thick carpet of healthy moss thriving on much of the roof.
Streamlines of this vintage have an unpolishable anodized finish, so the "care and feeding" is a bit different from Airstreams. There was one of every kind of metal polish in the various cabinets, slightly used. My initial brute-force cleaning involved slinging chlorox cleanser over it and scrubbing with a push broom. I am sure my neighbors were snickering behind the curtains, but an hour of that, and it was like a different trailer, just what the Dr. ordered!
To address the black coating without stripping the clearcoat, after a good scrub-washing, and spot-removing gummy tree-sap, you might try automotive polishing compound, or the more aggressive rubbing compound then polishing compound, followed by wax. Again, test small inconspicuous area first! The black is probably a "biofilm" of algae or something that lives off the dust and daily condensation.
It seems I love the mountains and deserts more than my friends do. I sure miss them!
1971 Streamline Imperial project "Silver Snausage", 1985 Coleman tent trailer, 1964 Little Dipper, 1975 Northwest "Proto Toyhauler", 2004 Harbor Freight folding, still seeking my Airstream.