What is Rousseau’s view of humans in civil society as slaves? Does this view, as argumentative and controversial as it might be, provide any assistance in trying to understand actual slavery?

In Discourse on the Origins of Inequality, Rousseau understands most people in society to be slaves of the few, the few being those who own the bulk of the property. This, he contends, creates inequality based on arbitrary rankings of people, which Rousseau argues does not exist in the state of nature. In nature, the only inequality is that of physical strength.
Once property came into being, the slaves resented the masters. Therefore, the masters created government in order to foster a sense of social unity, saying that they would protect the rights and property of the poor in exchange for the power to run society. However, since the poor, by definition, had very little wealth with which to ensure they were actually protected, the masters continued to exploit them. A key point, too, is that both masters and slaves were and are enslaved by this system, which warps humankind's innate nature in order to perpetrate an unjust social organization.
As for helping to understand actual slavery, one would have to define what "slavery" is, especially as slavery exists and has existed in many guises. The most obvious example, however, would be slavery in the southern United States before 1863, which, based on racial categories, allowed whites to buy, own, and sell blacks.
Rousseau's Discourse provides help in understanding two aspects of this slavery system: first, the institutional classes of slave and free were largely arbitrary, as categories of "black" and "white" had little to do with actual biology and everything to do with convention. For example, a person who was only one-eighth black—or, in common parlance, had only "one drop" of black blood—could be deemed fit for slavery. This accords with Rousseau's argument that inequality is not natural but socially constructed to serve the interests of the powerful. Second, many, many observers, including such disparate figures as Quaker abolitionist John Woolman and patrician slave owner Thomas Jefferson, noted the corrupting aspects of institutionalized slavery on both blacks and whites. Owners too were "enslaved" by the system of cruelty that warped their moral natures. Owners lived in fear of their slaves. Everybody ultimately suffered under this system, an observation which accords with Rousseau's notions of the effects of inequality.