Officers forced their way into the recreational vehicle parked in the 500 block of C Street in Chula Vista after no one answered the door during a welfare check.
His body was found shortly after 8:30 p.m. Sunday, according to Chula Vista police. Paramedics tried to revive the man, but he was pronounced dead at the scene.
Smoke from a small fire inside the vehicle was spotted during the welfare check. The man may have lit the fire inside the motorhome to keep warm and the flames likely smoldered due to a lack of oxygen, police said.
FOX5 learned there were no signs of trauma on the body indicating it was an accidental death. Officials are waiting for the San Diego Medical Examiner's report to confirm the findings.
Anyone with information is asked to call police at 619-691-5151.