The victim, whose name was withheld pending the notification of his family, was declared dead at the scene by medical personnel. He had suffered one gunshot wound to the torso, Scardina said.
It's unclear whether the dead man lived in the trailer permanently, but he stayed there on occasion, Scardina said. The two men who lived in the trailer, ages 53 and 56, knew the victim and were detained for questioning, Scardina said.
Officials later Thursday announced they had arrested Ken Neville, 56, of Dillon Beach, one of the trailer's residents. He was booked into Marin County Jail on suspicion of murder.
The nature of the relationship between the three men is under investigation.
Sheriff's officials visited the same trailer in July for a report of a domestic disturbance, Scardina said.
That visit led to one person being placed under a psychological hold, Scardina said.
In 2007, sheriff's deputies responded to another killing on the beach near Lawson's Landing. In that case, a Santa Rosa man was beaten to death following a fight on the beach.
Lawson's Landing is an oceanside campground just south of Dillon Beach. For decades, the owners have allowed people to park their trailers or recreational vehicles at the property on a full-time basis. However, that's set to change in the summer of 2016.
At that time, the California Coastal Commission will require that the long-term trailers be removed and that the land only be used for short-term camping, said Carl Vogler, co-owner of the resort.
Because of the coming change, Lawson's Landing now hosts far fewer full-time residents than it has in the past. Currently, there are just 22 full-time residents, and some are employees of the campground, Vogler said.
Two residents reported hearing one or two gunshots during the night, Vogler said.
He lives on the other side of the property from the trailer and didn't know about the shooting until he got a call from sheriff's dispatchers.
He said three people had been living since June in the trailer where the man was killed. He didn't know them very well, but said they were quiet. They didn't cause any problems aside from the time law enforcement was called out for an argument last year, Vogler said.
A younger person, possibly the 19-year-old, was at the trailer often but not listed on the lease, Vogler said. "He seemed pretty pleasant," he said.