Penguin if your trailer is in good condition I would not even consider using Zep floor shine on it. There are many products that are made just the gel coat. Here is one that I have used that works well, Duragloss Marine RV Polish #501.
I have been very happy with all of the Duragloss products that I have used and I am now at a point where I use nothing else on all my cars and truck.
A good website to visit is autogeek.net You can learn alot thru the forum there and can also order products.