Symbolism, Imagery, Allegory
To carry him through the U.S. in style, Steinbeck commissions a special truck with a little house on the back so that he can bring everything he might need and be able to sleep back there, if needed.
According to Steinbeck, he decided to name the truck Rocinante because his friends seemed to think his travel plan was as silly and, well, quixotic, as the famous literary hero Don Quixote: "because my planned trip had aroused some satiric remarks among my friends, I named it Rocinante, which you will remember was the name of Don Quixote's horse" (1.2.4).
Of course, Steinbeck thinks what he's doing is pretty important (at least, for him personally), but his truck's name is a little tongue-in-cheek acknowledgement of the fact that dropping everything in your life, leaving your wife at home, and just setting out on the road without a clear purpose (and as a middle-aged man) might seem (or be) a little... kooky.
In other words, the truck kind of symbolizes the wackiness and lack of structure in the whole trip, signaling that the journey is more about a general bee in Steinbeck's bonnet and need for a quest than some kind of clear and structured mission. And hey, that's okay—kudos to Steinbeck for being able to joke about it.