To supplement effectively means to complete or add to, and in The Grapes of Wrath by John Steinbeck, history and literature most definitely work together to create or complete the story being told.
The Grapes of Wrath is sometimes considered a work of historical fiction, and as such, the reader benefits from background knowledge and connections to the historical context in which the novel is set.
Works of historical fiction include ties to historical settings, figures, and events while also incorporating the literary elements one might find in any work of fiction. So, in effect, the argument can be made that history and literature supplement or complement each other in all historical fiction.
The Grapes of Wrath is set during the Great Depression, and the novel chronicles the Joad family's journey from Oklahoma to California in search of work. The Joads have lost their farm and must sell the bulk of their possessions due to the failing economy present during the Great Depression.
The historical setting actually sets the novel up for its most prominent conflict, and it is through this conflict that the reader is able to follow the Joad family's journey and witness the character growth that the protagonist, Tom, undergoes over the course of the story.
In this novel, as in most pieces of historical fiction, history and literature work in tandem and augment each other over the course of the story.