Question

Why do the trucks have metal chains at the bottom near the ground?

Answer 1

Gasoline is flammable, which means if a spark gets to it, it could catch on fire. A gasoline truck or any container that holds gasoline is vulnerable to sparks due to friction from moving the container.  So containers for gasoline and vehicles moving gasoline typically have a wire or chain to "ground" them.

Grounding something means to keep an electrical charge from building up, so it flows into the ground. It gives it someplace to go instead of sparking. 

So transporting that gasoline means you have a big chance for static electricity buildup.  The chain acts as your 'ground' and transmits static electricity harmlessly into the ground.