Did you try "marine" wax? It is made for gel-coat fiberglass. You should be able to bring back some of the shine, but it will be a lot of work and take several coats. I had a 2005 fiberglass camper and after 4 coats it had a decent shine, but that was just working one 3x6 foot area and it was work, I never finished it before selling.
Zep will make it shine like a new penny, but then 4-5 years later it will start to look like an orange peel and need to be re-done. If you do not mind redoing it then use the Zep.
