Well in the Michiana area the counties in Indiana are LaPorte, St. Joseph, Elkhart, LaGrange, Kosciusko, Fulton, Noble, Starke, and Marshall. It also says Lake and Porter, but that is the Region over there.
Well, since it is a fictional geographic area made up mostly by the media, there is no designated "Michiana" area by cartographers nor dictionaries. Given the term, it is fairly obvious that it is deriven from the MICHigan/indIANA bordering communities. Thus, the communities closest to the border between state lines are in general referred to as Michiana. Kosciusko Co. is part of my territory for work, and they are more linked with the "lakes" in Indiana than Michigan.
Lesson adjourned.