Peter (fl. 1863) (also known as Gordon, or "Whipped Peter", or "Poor Peter") was an escaped American slave who was the subject of photographs documenting the extensive keloid scarring of his back from whippings received in slavery. The "scourged back" photo became one of the most widely circulated photos of the abolitionist movement during the American Civil War and remains one of the most notable photos of the 19th-century United States.
As much as I hate to say something that has any possibility of being interpreted as defending Trump, I don't think he was trying to say that slavery was good (I'm referring specifically to his "museums talk too much about slavery being bad" comments from the other day). I think he was attempting to express, in his haphazard and thoughtless manner, a belief that is widely held among certain people, specifically white rural people. Namely, that focusing on the negative takes away from the positive.
Now, I don't agree with that sentiment but I do think it's helpful to understand the intent, or at least how his base is likely to interpret such comments. You and I probably share the belief that understanding history in all of its ugliness is necessary to both appreciate how we got to where we are and prevent us from making similar mistakes in the future. His base doesn't care about any of that. All they know is that talking about slavery makes them uncomfortable so they would rather ignore it. It's not as much about glorifying or whitewashing slavery as it is about avoiding discomfort. A lot of the time they will end up doing one of the first two in order to avoid the topic but that appears to me to be a simple attempt at justifying their desire to avoid the subject entirely.