can a union in a right to work state negotiate for a member benefit only

I am a labor lawyer and all labor matters (except for agriculture and a few other exceptions) are governed by federal law. It would be helpful to know more, but I can give you a general answer in light of what you wrote. The simple answer is "No."

In about half the states, a union and an employer may negotiate a clause in the contract which requires all employees in the bargaining unit to join the union or, depending on the union's success in negotiating, pay union dues even if the employee does not formally join. However, in slightly less than half of the states, such clauses are prohibited. These are the right to work states. They are primarily in the south and southwest.

A union always represents a clearly defined bargaining unit. The bargaining unit consists of particular jobs. That unit is usually certified by the NLRB after an election. The union must represent everyone in the unit, regardless of union membership.

If you are in the bargaining unit, then any benefit negotiated by the union must apply to you. If you are not in the bargaining unit, then the benefit does not apply to you.

If you think the union has not treated you fairly, go to the nearest NLRB office (you can find it at nlrb.gov) and you can file an unfair labor practice charge. You do not need counsel and there is no cost.