What is interesting about your question is that it goes against the usually-accepted anti-slavery theme of the book. In this regard, your question is a challenge, but definitely provable using incidents from the work.
One needs to look no further than the larger-than-life hero, Oroonoko, who is known as a "noble savage." It is the latter part of the term that proves your point more than the former part of the term. The fact that Oroonoko is considered "noble" would go against your point. However, the fact that Behn believes "primitivism" to be "good" and the fact that Oroonoko is still portrayed as a "savage" would be good proof of your point.
“He knew almost as much as if he had read much.”
“A poet is a painter in his way, he draws to the life, but in another kind; we draw the nobler part, the soul and the mind; the pictures of the pen shall outlast those of the pencil, and even worlds themselves.”
Through these quotations and, further, through Oroonoko's character, Behn shows that, even though she speaks of him fondly, Behn still considers her protagonist a primitive savage. Even eNotes admits that Behn's idea of "slavery is depicted as endemic in Africa." Endemic means constantly present and even natural for people of a certain class in a certain region. The use of the word "endemic" is telling itself.
There are two ironies that can serve as lesser supports. The first is a fairly obvious one: Oroonoko is a failure in regards to his struggle against slavery that is already engrained in a society. The second is also interesting: Oroonoko (along with his grandfather) actually sell their captives in the lower classes into slavery (even though Oroonoko tries to protect the noble ones). Truthfully, how can Oroonoko be said to be against slavery when he deals in slaves himself!
Finally, there is an interesting point hidden within a few details about Behn, herself. Behn takes great pains to convince us that the story is "true." Just look at the full title: Oroonoko: Or, The Royal Slave, a True History. At one point, we are to assume (as her readers) that Behn has been sent to glean info about a possible slave revolt. The irony here is that it is the white colonists who ask her to spy. Why would white colonists ask Behn to spy on a possible slave revolt if she, herself, was on the side of the slaves? It either makes little sense... or is very very telling that Behn is actually on the side of the Europeans.