[ineedasername]

No, a union contract supersedes a "right to work", because it's a contract. Yes, an employer can fire you for little/no reason (if it's not discriminatory) but if they sign a contract saying they won't fire you, then their hands are tied.

In the example of something like missing a day of work, the contract will spell out the specifics of advance notice for days off/sick days. If it's completely unannounced, the contract will spell out the steps of a disciplinary process.

Source: I work in a right to work state, used to be a union member, asked this very same question at the time.