My wife works for the census. They use aerial pictures and send workers to everything that exists- garages, building sites, solar panels, barns. All of them will be canvassed but nobody wants to work in the city.
It's worse than that. In Florida, if you have an ITIN number (issued by the Treasury Dept to anyone at all) you can get a state ID card which lists your home address as "I-275 Underpass at U.S. 301" (if that is the underpass where your cardboard box is located.) Then, with that same ID card, you can actually register to vote.