For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Steinbeck and his French poodle, Charley, pack up their stuff in a big truck named "Rocinante" and hit the road to check out America. And we mean really check it out—they're planning to go all over: north and south, east and—you guessed it—west.
Act II
As promised, Steinbeck heads up from Long Island through Connecticut and the rest of New England before heading back down and west. Along the way, he meets a lot of interesting folks and talks with them about all kinds of different topics, from nuclear submarines to hairdressing. He also meets up with his family and friends out in the area around Salinas where he grew up (in California). Then, he heads back east by swinging through the South.
Act III
The South is where this mostly pleasant and inoffensive journey takes a turn for the distinctly unpleasant and (way) offensive. He's traveling through the South during a time when race relations are super-tense and civil rights is the big topic, so there are a lot of ugly racial stereotypes and epithets dropped in conversations. Steinbeck is totally shocked by the nastiness of the emotions involved, particularly on the side of white folks.
He checks out a group of women, for example, who have made it their life's mission to protest the enrollment of African American children at a New Orleans school by cussing them out as they arrive at school. Steinbeck is totally shocked at the women's behavior—and still more appalled when he realizes how many people around there support and applaud the women's efforts. Gross.
Things calm down again (for him, not for the South) from there, and he drives back north to get home. He decides his journey is over—mentally, at least—in Abingdon, Virginia.