If your Imperial has a fiberglass roof it is oxidized.
I have a fiberglass roof, last year I took the time to completely scrub the roof using a concoction of Awesome, tide, and a little bleach. I used a long handle stiff brush which got +80% of mold/dirt off. I then used a small pressure washer to get more off and as need a stiff hand brush.
Got the roof as clean and white as it was when new.
I then applied 2 coats of Zip High Traffic Floor Wax with a long handle sponge roller (small diameter).
6 months later I was able to easily wash the roof clean and then applied 2 more coats of the Zip Wax.
2002 Monaco Windsor 38 PKD
2005 Jeep Grand Cherokee w/5.7 Hemi