What is Kolchin's main thesis in American Slavery?

I suspect there are two issues making it difficult for you to find a thesis in the book. The first is that a book is usually more complex than a shorter paper or essay and thus makes multiple complex points rather than a single simple claim. The second issue is that scholarly books are written to respond to an existing scholarly conversation. As much as making claims about slavery itself, Peter Kolchin is making claims about how other scholars have addressed the issue.

The central claim of the book is that scholars should study slavery in the United States against a background of forced labor in various colonies across the Americas rather than see it as a unique and isolated phenomenon. He also argues for seeing slaves neither as pure victims nor as purely creators of a subversive autonomous culture, but rather as somewhere in between. 

Not all elements of the book are argumentative though, nor is the entire book focused narrowly on a single thesis. It is also intended as a broad history of slavery.