This will be a tough one to answer, as we'd have to essentially prove a negative.

What I can say is that there is a sizable literature on how slavery depended on constant, widespread violence and terrorism to survive, how the sale of people was the ultimate form of dehumanization, and other aspects of slavery that were uniquely horrible. While there is plenty of literature on the plight of the urban working class in antebellum America, there is not any kind of historical school that argues workers had it worse.