Priebus won’t be able to stop Trump. He’s a paid employee chosen by some members. Most members will get behind Trump as the new head of the GOP.
Trump has been accusing the RNC of being crooked and corrupt. Why would the people he's been insulting back him? They have absolutely no reason to do so, because as we already know, Trump voters are much less likely to be involved at the party themselves at the grassroots level.
By deliberately (and proudly) choosing not to be involved with the party itself at the state and local level, most Trump supporters have ensured that they'll have very little influence within the party itself even if Trump does win the nomination.
Trump has gone out of his way to piss off the members of the RNC, and you think they would support him rather than Priebus??? More than 2/3 of them (which is what it takes to remove the chairman)?? Since they are selected by state members of the party (the "establishment"), Trump really has no leverage over them. So why would they get behind Trump?