I noticed the back of your project RV has the black "mold?" that forms after not being washed often enough. I've had the same problem with classic cars I love to restore. If sits for a long time the black is almost impossible to remove.
My RV was one of my "classic car" projects like yours. It had sat inside a barn for 10 years and had the same black mold. I tried rubbing compound and a power buffer. I tried diluted bleach which didn't work. My wife thinks the Great Value Bathroom Cleaner works better than Clorox or Lysol cleaners on a fiberglass shower stall. So I sprayed a 6" test circle and left it for 15 minutes. All the black magically disappeared with no scrubbing. I don't think it is good for the gel coat so I only used it the one time. Then a quick pass with polishing compound and power buffing restored the shine. Which has lasted 10 years so far with just wash and wax twice a year.
My RV was one of my "classic car" projects like yours. It had sat inside a barn for 10 years and had the same black mold. I tried rubbing compound and a power buffer. I tried diluted bleach which didn't work. My wife thinks the Great Value Bathroom Cleaner works better than Clorox or Lysol cleaners on a fiberglass shower stall. So I sprayed a 6" test circle and left it for 15 minutes. All the black magically disappeared with no scrubbing. I don't think it is good for the gel coat so I only used it the one time. Then a quick pass with polishing compound and power buffing restored the shine. Which has lasted 10 years so far with just wash and wax twice a year.