Fiberglass siding crack

It's likely a stress crack. The simple fix would be to drill a small hole at the end of the crack to stop it from spreading, then caulk it. The proper fix would be a fibreglass patch and paint but it will be hard to match and blend it in.

Definitely a stress crack from the slide out. My last TT did the same thing, only it was alum sided. Not an easy repair to make it look like new again. Slide outs are great but they put a lot of strain on the shell and structure of the trailer. Even if you fixed the crack and refinished it, it might open up again if it moves when the slide out is opened. Drilling a hole, like mentioned, usually will stop the crack, sealing it up and keeping an eye on it might be the best remedy for now.