In what chapter of The Grapes of Wrath is the slot machine?
There are eating places all along the road. Their favorite customers are the truck drivers who pay for their food, leave a tip, and bring other customers. The wealthy people who stop constantly complain and spend little. While two drivers are in one such place, an old car loaded with family possessions pulls in and the man in it asks for some water. He then asks if he can buy ten cents worth of bread. (eNotes)
Chapter 15 is a rather long non-narrative chapter dealing with three classes of people, a roadside diner, and the social/political environment that prevails in the interstices.
The diner has a slot machine and the owner of the diner tracks the payouts so that he can predict when they will next payout (give money to the player). He uses his information to keep people from winning.
While this is true, the man is not entirely ungenerous. He instructs the waitress at the diner to sell a fifteen cent loaf of bread to a poor migrant family for ten cents. He also expresses admiration/affection for the truck drivers who frequent the diner.