It's been awhile since I've dealt with scholarship on the Dust Bowl, but I'll try and answer your question.

Really, looking at Steinbeck alone is a little ineffectual. He was part of a cadre of social critics, all of whom turned their attention the Dust Bowl Migration at the same time. The other major names are Carey McWilliams (author of Factories in the Field), Dorothea Lange, and Paul Taylor (who captured and produced the 'Migrant Mother' photo and others). Their works really impacted the early direction scholarship would take, but all them tended to cast their subjects differently than how the Okies(1) perceived themselves.

In particular, they tended to 'prolelarianize' the Okies. Though the California Central Valley was and is a hub for agricultural agitation, the migrants were very divided when it came to participating in labor disputes. Some scholars have attributed this to anti-communism, some have suggested that the migrants wished to join the 'grower' class at some point, and decided against agitation.

Steinbeck, McWilliams, and Lange also deliberately played up the rough language and living conditions of the migrants. Regional accents and 'color' were in vogue, and thus the contemporary writers emphasized this where possible. This is where a number of the criticisms of The Grapes of Wrath came from (on the Okie side of the equation, the growers are not exonerated), as it was portrayed them as somewhat crude in an already vulgar story, when they strove to maintain dignity, cleanliness, and a strong sense of self-worth. That criticism would be two-faced, in a way. While some migrants did take issue with the language for those reasons, growers used the vulgarity to push for the book's banning in public libraries.

Aside from those notes on Steinbeck and his contemporaries, I'd suggest reading through the Gregory book below. It should answer most of your questions.

(1) The word 'Okie' is now, in a sense, owned by the former migrants. This is confirmed in the scholarly works below, as well as my personal experience growing up in Bakersfield, Ca, and my experience in interviewing and transcribing interviews for the California Odyssey Project at CSU Bakersfield.

Gregory, James. American Exodus: the Dust Bowl Migration and Okie Culture in California. New York: Oxford University Press, 1989.

Weber, Devra. Dark Sweat, White Gold: California Farm Workers, Cotton, and the New Deal. Berkeley and Los Angeles: University of California Press, 1994).

Shindo, Charles J. Dust Bowl Migrants in the American Imagination. Lawrence: University of Kansas Press, 1997.

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