Magill said Joseph Sizemore, 42, had been reported missing by his family on Sept. 7.
He was last seen that day on Washougal River Road when the red 2009 Ford Focus that he had just purchased ran out of gas.
"We don't know how Mr. Sizemore ended up in Mosier," said Magill. "We talked to his family and no one could really explain it."
According to reports, Sizemore called his girlfriend to let her know that he was out of gas shortly before noon on Sept. 7 from the Washougal River Mercantile store.
Skamania County deputies were notified and provided him with five gallons of gas.
Sizemore reportedly told them that he was headed home to his residence on Northeast Halsey Street.
He never arrived at that location and Skamania County authorities launched a search for the missing man, who was entered into a national missing persons' database.
A description of his vehicle was made available, which still had temporary tags and Portland Nissan signs in the license plate holder.
Family members told media sources that Sizemore was schizophrenic and had not been taking his medication.
Magill said the man's vehicle was found Dec. 14 at 2350 Carroll Road in Mosier, the site of the former American Adventures RV Park.
The sheriff's office received a report that the car had been parked in that location for a long time.
Upon investigation, deputies connected the vehicle to Sizemore and a search of the area was initiated in cooperation with Skamania County authorities.
Magill said the search began about 1 p.m. Dec. 15 and Sizemore's body was found in a ravine about one mile to the north of the RV park.
"There was no indication of violence or a self-inflicted injury – no weapons were found in or around the vehicle or near the body," he said.
Sizemore's body was turned over to the Oregon state medical examiner.
According to Magill, the decomposition of the remains suggested that Sizemore had been in the ravine since his Sept. 7 disappearance.
"For some unknown reason, he might have decided to continue with his drive in the gorge and somehow made his way there," said Magill.