A man believed to be responsible for some of the 20 arson fires that have scorched vehicles in the San Fernando Valley was arrested Tuesday after a motorhome was set on fire and someone tried to ignite a truck. The 34-year-old suspect, whose identity was being withheld, has a criminal history and is not believed to live in Los Angeles, said David Liske, a senior arson investigator with the Fire Department. No information about the criminal history was released. The man had been a person of interest in the series of fires even before the two latest fire were set, Liske said.