I agree with the others here, an RV rental would be really expensive (and personally, a nice air conditioned hotel room with a comfy bed each night is my preference)
May I suggest another alternative, book your flights as an "open jaw" or 2 one-ways. By this I mean fly in from Australia to CT then make the return flight from Orlando (or somewhere in Florida). This will save you having to back track all the way to CT at the end of the vacation.
Then book a one way car/SUV rental between the CT and the Florida airport (MCO = Orlando).
Pick out your points of interest along the route.
Book a night or two hotels based on your POI's
Last few days spent at WDW and then fly home from MCO.
We frequently plan this kind of trip as we like the "roadtrip" part of the vacation, but hate having to backtrack at the end of the vacation (the end of the vacation is depressing enough without having drive a bunch to take a flight back home).
This Feb, we flew into Miami, went on a cruise, picked up a rental car at MIA and drove up to Orlando for the "Parks" and flew home from Orlando.
Last year we flew into New Orleans, spent a few days in the area, then drove the rental car to Orlando (with a stopover in Panama city). Spent 10 days enjoying WDW, USO, Daytona Beach etc then flew home from Orlando