As Carol says sometimes the combination buff and wax "restore" products can yield better results if followed by the complimentary wax only product from the same manufacturer. Two things reduce the shine, chalking of the surface and pitting which make the surface less smooth and reflective. If you have removed the chalk by rubbing it out then a second coat of wax may fill in the pits.
I can sum up the con of floor wax on fiberglass.
1). It penatrates and prior to painting
total removal is required to avoid "fish eyes" from the reaction of paint
2). The long term track record of both Zep and RM is unknown for this use. UV resistance, repeated application results, and the most effective process for total removal are less established than say paste wax.
The first is a given for any wax type product but since few have used the floor wax and then painted their trailer the best process for doing that is not really well established.
The second concern is getting reduced the longer people use these products and can provide feed back on real world experiences.
Poliglow has the same issues on removal for painting
. It's pretty close to the same product just diluted and more expensive.
Use the link Tom provided and you will find a wealth of information and discussion to help you decide and implement.
I'm really happy with the results of RedMax (as is DW) but my trailer is 34 years old and the white gel coat without the RM is so pitted it is slightly rough to the touch and wearing thin in places, I can't rub it out to a smooth surface. Now it's all shiny and looks almost new. I would also add if your going to replace decals or stripes do it first so you stick them to trailer not the wax.
Newer trailer with less wear and age on the gel coat such as yours or Carols could have other options that would yield good results.