The US Census is "de jure" (based on where you live and sleep most of the time), not where your mail goes, so the SD nomad population can go uncounted. The Census Bureau generally does not mail forms to PO Boxes. They use a Master Address File (MAF) based on physical residential structures. If you are an RV'er and you were at a campsite in Arizona on Census Day (April 1st), you were technically supposed to be counted there, not in South Dakota. Many truckers are "transient" and difficult for the Bureau's "Non-Response Follow-Up" (the people who knock on doors) to catch.
Many are not counted. This creates a fascinating paradox: South Dakota has a high number of legal residents (on paper/licenses) but a lower enumerated population (on the Census). South Dakota might have enough "licensed residents" to clog their DMV and insurance systems, but because those people weren't "counted" in the physical state during the Census, the state doesn't get the federal highway or healthcare dollars to support the infrastructure they use when they do pass through.
It’s a bizarre irony: In PNG, the government "invents" people (overcounting) to get more aid; in South Dakota, the system "loses" people (undercounting) because the administrative tools (physical addresses) don't match the modern lifestyle.