Several years ago at the Tampa RV show I stopped at a vendor booth that specialized in replacing the sides of dark painted motorhomes. He said that in some coaches the fiberglass under the dark paint was "toast" in as few as 5 years. Take the handy IR thermometer you use to check your tires (you do use one don't you) and measure the sunny side of dark, beige, and white. It will vary by more than 40 degrees. Something has to handle that extra heat; insulation, air conditioners, or a combination. The laws of thermodynamics have not been repealed.
It's probable that fiberglass manufacturers have manged to change their formulation to survive the extra heat else we would see numerous threads on fiberglass replacement. I would carefully inspect a dark coach over a half dozen years old.
I was lucky to find a pristine unpainted 2014 a couple of years ago. Rare find. It's worth noting however, that white is not a guarantee of trouble free fiberglass as seen in the now defunct Alpha line.
