From the Rules of the Republican Party
RULE NO. 11
Candidate Support
(a) The Republican National Committee shall not, without the prior written and filed approval of all members of the Republican National Committee from the state involved, contribute money or in-kind aid to any candidate for any public or party office of that state, except the nominee of the Republican Party or a candidate who is unopposed in the Republican primary after the filing deadline for that office.
So if there's a contested primary, the RNC can't provide financial support to any of the candidates.
Whether this counts as "neutrality" is a matter of semantics. It also prohibits the RNC from subsidizing all the candidates equally, which would still be neutral.