Either 12 volt or 120 volt lights can be used to illuminate the perimeter of the trailer. 120 volt lights can provide more light, but you'll need to have the generator running or have your shore cord hooked up first. 12 volt lights can run off the battery in the trailer.
Keep in mind that stairways, doors, etc. that will be used after dark should be lighted for safety.
Running lights are the red and yellow marker lights that highlight the outline of the trailer at night as well as the brake, turn signal and tail lights. These are mandated by federal DOT regulations, and trailer manufacturers are required to install them to those requirements.