At the start of the journey westward, the Joad family is intact: Grandpa, Grandma, Pa, Ma, Tom, Rose of Sharon and her husband, Noah, Al, Ruthie, and Winfield are all there. Before they are out of the state of Oklahoma, Grandpa dies. Soon, Noah decides he doesn't want to leave the river and he stays behind. Then Grandma dies. Rose of Sharon's husband takes off when he realizes things in California aren't as good as they'd hoped they be. After Jim Casy dies and Tom's injury would identify him as a killer, Tom leaves the family. Next, Al decides to stay with the Wainwright family and marry Aggie. The family members left are: Pa, Ma, Rose of Sharon, Ruthie and Winfield. The family is cut in half. While this disintegration of the family is happening, the family's economic situation which is bad to begin with, declines as well. When the journey began, the family at least had some food and a truck, but by the time the story ends, they are destitute and have gone into a stranger's barn to try to stay dry because they have no where else to go. They have no truck, no food, and no money.
This is a great question. I like that you are comparing the economic decline during the Great Depression with the problems the Joad family faces during the entire novel.
My analysis of this is that the economic decline, caused by the Depression, is what leads to the disintegration within the Joad family. Think about it, the family was doing just fine before the Depression, before the farming drought, and before the dust bowl hit. The family was living happily on their farm, as many Midwesterners did up until the storms hit. However, as the farming industry went down the toilet and the Depression came into full force, families and farmers began to lose their jobs and thus, their income.
This loss in income and work, leads the Joad family across Route 66 to California, in search of any possible work they could get their hands on. Unfortunately, as we read throughout the book, this is also what sends the family through various struggles and problems. Death, hunger, and feuds within the family are all what is experienced throughout the book, and it is tied directly to the down-spiral of the economy. Without this fallout in the economy, the Joads wouldn't have needed to drive across the country in search of work, and thus would probably not have experienced the problems that they did.