I have personal experience with a gray colored 5th wheel and I would never do that again!
Two reasons stand out for me...first, the gel coat faded (not oxidized) after a very short time (12 months) and continued to worsen the entire time I had the RV. UV plays havoc with colored gel coats and unless something has changed (improved) don't expect it to stay color-fast for long.
Secondly...this may be my imagination but I swear the darker color made the inside warmer on those hot summer days.
They do look nice for awhile though!