A ventilation system for a trailer at a loading dock includes a fan supported by a positioning mechanism that can vary the extent to which the fan protrudes into the doorway. In some cases, the fan can protrude into the trailer itself to ensure airflow to the full depth of the trailer. When not in use, the fan can retract to a stored position where the fan is completely clear of the doorway, so the dock door can close. In some embodiments, the positioning mechanism includes an articulated arm with various pivot points that enable the fan to aim into a trailer at different angles and even move to another doorway to ventilate a different trailer. A lamp can be mounted to the fan and/or the fan's positioning mechanism.