Annual "Smog Check" on Diesel Motor homes, where?

If a city exceeds ozone levels and ends up on the list, any contiguous county is also added. I see the need in places like Dallas, where Collin county is almost as densely populated as Dallas county, without a large population center in any one city. If San Antonio were to fail (and they have come close several times) four or five very rural counties will be added to the list along with Bexar county.