The plastic is relatively soft, and in warm weather or direct sunlight, it is especially vulnerable. Microgrit can then leave almost invisible scratches in the material when a mitt or brush is used.
However your description implies a color change, including the section above the windshield, which as far as any I've seen does not have plastic film over it - since you describe the change as different above than below, I would surmise there is no film above. It may be that you got into some tree sap that smears more than it comes off, or you used a solvent during cleaning that had an effect on the film. I've used some products that leave a difficult-to-remove oil-like residue, visible more at certain angles than others.
Wash in cool conditions, out of direct sun, and use only mild soaps approved for automotive finishes. Most other common household detergents can be harmful, especially to older fiberglass which it can oxidize and leave a film on. If your paint is automotive over fiberglass, it is less susceptible, but detergent should still not be used. Monaco approves only Dawn, though other automotive-dedicated products work.
When on long trips, I apply 303 Aerospace Protectant or Thetford Protect and Shine UV inhibiting product to the front cap and mirrors every couple of weeks. It fends off solar deterioration of film and paint, and helps bugs rinse off easier, especially the Thetford product. But for ease of final polish, apply them morning or evening to a relatively cool surface, and out of the sun.
Joel
