While traveling incognito through the country, Hank and King Arthur are sold into slavery, and chained onto the same gang Hank had first seen in Chapter 21 -- you see them here, at the end of coffle. The chapters recounting their experiences result in the novel's most sustained engagement with slavery, and at one point, at least, Hank's narrative overtly connects their sufferings to American circumstances: he points out that the law that allows him to be enslaved is the "same infernal law [that] had existed in our own South in my own time," which required freed blacks to prove they were not slaves.
Hank also thinks of American slavery earlier in the journey he makes with the king, when he sees how willing the peasants are to serve the lord of the manor against their own class and interests. It reminds him of the "'poor whites' of our South," who fought for the "slave lords" in the Civil War, dying to preserve slavery despite the way slavery was "the very institution that degraded them" as well as the slaves.
The Barrett Collection, UVA PS1308 .A1 1889