The Fatal Environment by Richard Slotkin is as much a work about how we study the history of the western frontier as a book about the frontier itself. It does not so much articulate a thesis about the physical geography of the west or the experience of western pioneers, as examine how a racialized ideology determined the way that the frontier and the people of the frontier were portrayed in literary works. Thus as you create a thesis for your essay, you want to focus on Slotkin's analysis of ideology, rather than a description of the frontier itself.
For a thesis statement, you could make a claim about whether you think Slotkin's analysis is correct or not, or apply Slotkin's method to other literary works that he does not discuss.
In this assignment, a successful thesis should demonstrate that you understand how power and racist forms of imperialism shaped the myth of the frontier, rather than focusing on the actual process of westward expansion.