The Russians most certainly did hack the RNC, but the fact you have not seen any RNC emails, but have seen DNC emails, is troubling.
If the Russians approached the Trump folks (through cut outs), they likely told them something like "a friendly entity of ours has some troubling information on your rivals." They would say this friendly entity will soon disclose this information. In the same conversation, they would be informed "they also came to a supply of information from your parties as well", but reassured "dont worry, they will see to it this information will be protected".
This would be the basis for an influence campaign by Moscow, in addition to any other kompramat already posessed. Mueller isnt ignoring the RNC emails, its just that there is a hotter trail on the DNC emails as they were disclosed and the RNC ones were not. Weve seen the "Podesta emails", but not the "Lewandowski emails".