A 7:15 PM flight ....... when we have had RAC handle our luggage, it needed to be there at least 3 hours before the flight time. HOWEVER, I believe RAC used to close in the early afternoon, so you need to find out about that. We always had to have it there by mid-morning, so never paid a lot of attention, and it may have changed. If you're planning to go to the parks that day, you can bring your luggage to RAC in the early AM.
RAC ships luggage on a truck that is separate from the ME bus, but you always have the option of handling your own luggage. The ME bus driver will load it onto the bus when you board and you can check it in yourself when you reach MCO.
Hope you have a wonderful trip!!