To answer this question effectively, it would help to define which period in history is the main focus. American literature can begin as early as Christopher Columbus's writings about discovering America and span over 500 years. However, common themes revolved around survival on land and with native tribes, then moved on to the ever famous phrase of "The American Dream" where anyone can pull himself up from his bootstraps and become whatever he wanted to become. Many writers discussed things like slavery, Manifest Destiny, the frontier, wars, battles, and the strength of the American spirit through all of the trials. There are also revolutionary political speeches and documents that that inspired topics on which to write. For example, the Declaration of Independence, the Constitution, and presidential speeches discuss themes of freedom, liberty, hard work, and a belief and dependence upon God. These themes have shaped debates, law suits, society, and education throughout America's history and continue to do so today.
American Literature covers different periods of time and the theme usually changed due to the time period. Most early American Literature centerd around the dream of having a successful career and family( "the American dream"). The Rise of Silas Lapham is a great example of this. Most fiction during this time was considered "historical fiction or realistic fiction" because it did not venture far from what could happen in real life. As time moves on, other themes arise that have to do with what is happening in the world, such as slavery in The Adventures of Huckleberry Finn. Other novels such as Moby Dick focus around adventure, but are still realistic fiction.