To make a nation-wide accuracy map, you'd need to compile property data from every county (or equivalent) in the nation. Assuming every county publishes it publicly (I'm sure some don't), it would still be a ton of work...

The real catch is this, though: if you actually pulled it off, you would have just built a nearly flawless reverse geocoder, and you wouldn't need to use an external API at all. You could just look locations up in your huge property data set.