I noticed semi-truck trailers are usually tagged in Indiana.
Many of the cars driven by Hispanics here in Iowa are tagged in Nebraska or Minnesota. There is something about certain counties there that make it easier to register or is an easy source of income for those counties.
I can remember when working construction, the cops would stop us and if we lived there more than a year, strongly encourage us to get local tags.
There may be an above average number of Indiana tagged trailers. I pulled semi trailers for almost 20 yrs. They were tagged with the states that the company was based: Illinois and Tennessee.
Springfield, MO, has several big trucking companies. I don’t recall seeing Indiana on any of their trailers.
I see the semi’s with Indiana or Maine.
Also Maine has a good deal for commercial trailers.