I don’t think Nina Revoyr’s premise is even true. I don’t go camping so I can’t say anything about campsites or RV parks, but in the southeastern USA you will see plenty of black people in parks and on hiking and walking trails. I do a fair amount of walking and hiking in various places and black people are out there. Nina’s “evidence” may be taken from areas with smaller black populations.
Here along the NJ/NY border the parks generally don’t offer much in amenities; no problem if you like nature but not something that attracts people looking for someone to tend to their every need. Basically once you get away from the parking areas it becomes very white and Asian (hordes of them come in vans to pick berries and such - I’m truly impressed by how some older ones hump pretty far into the woods).