Last I heard, the RNC has spent NO MONEY on Trump ads!!! Why should he spend his time raising money for THEM!?!?
Turns out both candidates have ended the fund raising phase of the campaigns. To your question though, the reason Trump helps the RNC is part of the usual business courtship. The opening is a gamble for both sides. He didn't have to invest much, a week of time maybe and some of his "political capital." What he is really looking for is the get out the vote apparatus. He has opponents in the party on that count too, Bush in Florida and Kasich in Ohio. I don' think the RNC is all that powerful or useful for GOTV, but I have no hard evidence (other than McCain and Romney losses) to back that up.