Boy, talk about a common problem. Tons of information about this on the Internet along with videos on youtube. Adhesive remover from Lowes or Home Depot will remove the glue and I have never had it harm the fiberglass.
The easier way I found to remove the decals is using one of the eraser wheels made for this purpose. Just chuck it into a drill and it works like a pencil eraser and feels much like the same material. If the decal is very loose you can use plastic blades that look just like the metal ones you put in a paint scraper. I got my disk at a local store which sells paint for automobiles, but you can also order them on Amazon.
Just google "removing decals from an rv".
You can use a heat gun, but dial the temperature down and keep it moving as it is possible to damage the fiberglass with too much heat.
I did not find removing decals was very difficult, but it is a little time consuming. If you take your time, I am sure you will do fine.