Grapes: A treatise?

The Grapes of Wrath is, at least in my opinion, easy to see as a novel and not a treatise. Here is the difference:

A treatise is defined as a work "formally and systematically" (according to Google) dealing with a subject. This is more along the lines of a newspaper article or essay on a subject to bring to light the underlying principles, or creating a criticism of it. In this case, The Rhetoric of Fiction is more of a treatise than The Grapes of Wrath.

A novel is defined as a work of fiction representing a character with "some degree of realism" (also Google), which by far fits much nicer with The Grapes of Wrath than a treatise. The novel, though it may have some bits of treatise in it, can be real, as if another world was created from our own.

Keeping this in mind, what characteristics does The Grapes of Wrath have of a treatise?