Your rationale does not hold up in most states won by Cruz, with Trump winning in the conservative, less urban or rural counties and Cruz winning in the more liberal, urban counties.
Why is that?
When voters are asked to identify, Cruz does best with very conservative voters in pretty much every state.
When you look geographically as you have asked, then you find less educated voters in rural areas that are also conservative.
I am not trying to insult anyone. The less educated voters vote for trump and they live in rural areas.
That does not change the polling about self identification that shows who very conservative voters are choosing apart from education or economic position.