I checked few things and below are my findings:
1. only citizen can vote but in some areas in local elections non citizen can vote as well. (not sure if that is why its not considered as proof)
2. What are 3 acceptable forms for proof of U.S. citizenship?
(1) A U.S. passport, including a U.S. Passport Card issued by the Department of State, without regard to any expiration date as long as such passport or Card was issued without limitation. (2) A Certificate of Naturalization. (3) A Certificate of U.S. Citizenship.
looks like that is why they are asking you these docs.
Was your ex a citizen at the time of marriage or he got naturalized later?
obviously the officer is not very cooperative and if there is no way you can find these data, I will say respond to the rfe and explain them why you do not have those details and if you can't get naturalized now you will try at 5 yr rule as you have very less time. Also see if you can check with any lawyer.