Answer by taking a quote from the following article:

Stewie Griffin: The Untold Story is a 2005 direct-to-DVD animated comedy film set in the Family Guy fictional universe. Released on September 27, 2005, the film's main plot point concerns Stewie Griffin trying to find who he thinks is his real father after seeing the man on TV. He travels to San Francisco, only to find that the man is him from the future.

Peter buys a TiVo. While watching it, Stewie spots a man in San Francisco on the news that has the same face and hairstyle as him. Stewie then believes that he may be his true father. Learning that Quagmire is going on a cross-country tour in which he plans to have sex with a different woman in every state of America, Brian and Stewie hitch a ride in his RV. At a motel in New Jersey, Quagmire is handcuffed to a bed and mugged by the latest woman. Then Stewie and Brian drive off with his RV leaving Quagmire at the motel.  Meanwhile, Peter and Lois are trying to get intimate, but are constantly interrupted by Chris and Meg. To solve this problem, Peter and Lois decide to teach the children how to find dates. After several "lessons", Peter and Lois send them to the mall. However, Lois is concerned that people will think they're bad parents simply because they wanted their children out of the way.  Stewie crashes the RV in the desert after going insane from ingesting an entire bottle of "West Coast Turnarounds". After wandering through the desert, Stewie breaks down crying and nearly decides to give up until Brian encourages him to keep going. The two manage to get a rental car and arrive in San Francisco. Stewie mysteriously leaves Brian and confronts the man from TV on a cable car, and is shocked to discover that the man is actually himself from 30 years in the future.

Where do they hitch a ride to?
San Francisco.