Richard Mentor Johnson was my 3rd great grandfather. I have read that Julia Chinn lived as his wife and I know they had two daughters. I have also read that it was rumored they were secretly married, but of coursre it was illegal at the time for a white man to marry a mulatto. My lineage is through their daughter, Imogene, who married Daniel Pence. About two years ago I was able to tour the home Imogene and Daniel built on the property Johnson gave them when they married. The house and family graveyard are on private property and in general disrepair, but listed on the National Register of Historical Homes.
I have a tree posted, so if you are interested in looking at it, you should be able to find it by searching for any of the above names.
You asked about the location - it was Scott County - Georgetown, Kentucky. Johnson was VP under Martin VanBuren. He gained national fame as the man who killed Tecumseh during the War of 1812. Little seems to be known about Julia Chinn other than she was a slave RMJ inherited from his father, Robert Johnson. I have read that she was raised by Mrs. Johnson. She died around 1833 during a cholera epidemic.
If you have any further questions, I have a new e-mail address firstname.lastname@example.org