Now that I have tracked great great grandpa John back to his childhood with federal census records, I can see if any state census records are available. I went to FamilySearch.org (https://beta.familysearch.org/) and found three South Dakota state census records for him, 1925, 1915, and 1905. The writing is a bit faded on the 1925 enumeration sheet:
On the 1915 enumeration sheet, we find even more information:
The 1905 enumeration sheet confirms the information from the other two years:
I know that South Dakota conducted a state cenuses in 1895 and a special Dakota Territory census was conducted 1885 (thanks to a handy-dandy little book called The Family Tree Resource Book for Genealogists which can be found at Family Tree Magazine's store: www.familytreemagazine.com/store), but I haven't had any luck finding either. It appears that the 1895 census only included 6 counties, not Yankton. The 1885 census appears to not include most of the southern part of the territory (which, of course, includes Yankton County). Looks like I'm going to have to find another way.