A Boring Dystopia

That's not really how it works, the census bureau is extremely thorough - they send people into encampments regularly, work with homeless charities of all kinds, etc. These counts are estimates unless it's a federal census year (when they absolutely do count every individual person that they possibly can), but they're not going to be wildly inaccurate.

The much bigger issue is that these numbers appear to be limited to city limits or greater city area, and that's where the discrepancy is gonna show up. Most homeless people dont live in cities, and camps are often established on conveniently unincorporated land so they dont have to be counted. Bureaucratic bastardry at its finest.