“You show me where the N.T. condemns slavery and doesn't explicitly endorse it.”
Several issues here:
i) As I’ve said on more than one occasion, Biblical law is not utopian. Biblical law is adapted to a fallen world. Biblical law is adapted to the socioeconomic structures of the day.
So Biblical law often involves a practical compromise between realism and idealism.
ii) NT writers weren’t revolutionaries or anarchists. They realize that law and order is essential, even though the state may be corrupt in varying degrees.
iii) As one scholar points out, “The ancient Hebrews as a people knew slavery in their Egyptian bondage (Exod 1:10-14; 5:5-14), from which they eventually were led to be free people under Moses (Exod 12:37-42). Because of that experience, Mosaic legislation developed certain rules about the keeping of slaves: ‘Remember that once you were salves in Egypt and the Lord your God redeemed you; that is why I give you this order today’ (Deut 15:15; cf. Lev 25:42-45,55). Even though slavery as a social and economic institution was recognized in ancient Israel, there was a clear attempt to humanize it in a way that set Israel apart from its neighbors. The social and economic structure of ancient Palestine was not, therefore, built on slavery, as it often was in other contemporary cultures and lands,” J. Fitzmyer, The Letter to Philemon, 29.
iv) This stands in contrast to Southern slavery. The agrarian economy of the Old South was labor-intensive. Slaves were an easy source of cheap, mass labor.
As such, Southern slavery is at odds with the aim of OT law, which attempts, as much as possible, to curtail a slave-based economy.
vi) Finally, as Richard Bauckham has documented in “The Economic Critique of Rome in Revelation 18” (The Climax of Prophecy, chap. 10), the NT does attack an economic system which is dependent on forced labor.
By analogy, that would also apply to Southern slavery.