Quest
By naming his truck Rocinante, Steinbeck automatically aligns himself with the quester to end all questers: Don Quixote. You know, that's the dude who was so committed to his trip that he embarked on it, and then continued on it, despite everyone else pretty much thinking he was a lunatic.
Plus, Steinbeck spends the whole story on a trip, so is it really so surprising that we would categorize it as a "Quest"? We didn't think so. Sure, Steinbeck isn't enflamed by some kind of divine or romantic purpose in making his journey (as many questers are), but he has a goal:
My plan was clear, concise, and reasonable, I think. For many years I have traveled in many parts of the world. In America I live in New York, or dip into Chicago or San Francisco. But New York is no more America than Paris is France or London is England. Thus I discovered that I did not know my own country. I, an American writer, writing about America, was working from memory, and the memory is at best a faulty, warpy reservoir. I had not heard the speech of America, smelled the grass and trees and sewage, seen its hills and water, its color and quality of light. I knew the changes only from books and newspapers. But more than this, I had not felt the country for twenty-five years. In short, I was writing of something I did not know about, and it seems to me that in a so-called writer this is criminal. My memories were distorted by twenty-five intervening years. (1.2.1)
So, there you have it: even if it was just a general mission to reacquaint himself with his subject matter, the story contains the kind of clear goal in a journey that defines the "Quest" genre. Also, he certainly encounters lots of obstacles (getting lost, flat tires, a sick dog, horrible racists) that impede him, and those sorts of hurdles are also staples of the quest.