The blaze in the 400 block of Van Houten Avenue in El Cajon was reported about 9:45 a.m., according to Heartland Fire & Rescue.
A 17-year-old area resident who attempted to extinguish the flames before emergency crews arrived suffered a cut arm and was transported Sharp Grossmont Hospital, a dispatcher said. No other injuries were reported.
It took firefighters less than 10 minutes to subdue the blaze, which destroyed the trailer.
The cause of the fire was under investigation.