I read a few articles, which you can Google yourself, but apparently no one really knows what happened to Solomon Northup after he reunited with his family, helped the underground railroad, and went on speaking engagements. His house was supposedly foreclosed on, and there is one article that said he became a vagabond, but the actual reality is that no one knows and probably never will.
I have submitted my theory that he may have been the one who traveled to an undisclosed location in South America to establish a colony for people who wished to disappear. Or maybe it was already established and he took on a position there? I suspect both Amelia Earhart and Ambrose Bierce know.