You've already sent them everything you had to show this was a bona fide marriage. They found that evidence to be insufficient. Therefore, I don't see how you can generate what they want to satisfy the RFE since you probably have nothing else to show marital union since the time you filed. However, failing to respond to the RFE at all will trigger a denial. I am not an expert, but I think filing for a divorce and amending your I-751 would be the best response to the RFE. The goal will be to prove you entered the marriage in good faith.