They will run a bus until all guests are out of the park, even post-closing dining. It may not be dedicated and could stop at several other resorts, depending on how many others need a ride as well. They'll probably ask you your resort as you leave and direct you accordingly.
Last year we left RFC decently after park closing. Not a single other guest was around. Went to the bus area and a CM out there asked where we were going, we told him and he told the driver on the nearest bus to take us there. Private bus! It was awesome.