More often than not, No, but the trailers are. the wind deflectors on the side / roof are, the side of unit's have a fancy trim are Fiber, some trailers sides are fiber, the only ones I have driven in the past, the likes of the old Scammels, that cab was total fiberglass.
The roof I an referring to is the roof of the box that has been taken off the back of an old lorry c1980's. I think that it was a complete lorry without a separate trailer.
I am looking at using it for storing some hazardous materials and would like it to be fireproof. My other option is a shipping container.
If you happen to have a spare/broken section of the box. Just put a match to it. You will soon find out if it's fireproof.
From a security aspect Fibreglass is also fairly easy to break/cut through, whereas a shipping container is'nt. Just another point to be taken into consideration when storing hazardous materials.
The Container should be your best option regards, safety, It has 2 opening doors at the rear with a locking bar on each door, If you use a good Lock, they only way you can open them is with a Bolt Cutter, the containers are a sealed Vented Metal Box with ply board floors at times, also incorporated in the floor / sides strapping point, I used to carry the 20fts + the 40fts. use the net to purchase, depending where you are the unit / container trailer will del, but ring around Enfable.