"I was thinking more of the two well-known facts that slavery was only in the South and that slavery ended after the Civil War."

I don't believe that there was any propaganda effort to support or coverup these ideas. But most people accept that slavery was outlawed in nonslave states, when it was being perpetuated in the south, and that slavery did end after the Civil War, replaced eventually by Jim Crow segregation laws in the south and de facto (social) segregation in the north, something we still have everywhere. Still I don't know of any propaganda efforts by the federal or state governments to cover any of this up. And that was the point.