There are no roads in Walt Disney World that are county controlled.
RCID maintains and owns the majority of the roads, including all of the major transportation corridors except the state highways and interstate 4. Disney owns the roads that are within the resorts, the parking lots, that approach the themeparks and most of the backstage roads. RCID has a map showing which roads they own. The RCID roads could be used by Universal skinned busses, however, all they could do is crisscross the property. They could be tresspassed from entering the resorts. Not saying that is a good idea, but it is a possibility.
However, consider that Mears has contracts for the Disney Cruise shuttle, many private event transportation and other on-property shuttle needs. They would never want to lose those contracts just to take minor ad revenue for skinned busses. Other considerations could include does Mears want to be the "official transportation provider" for Walt Disney World such as has been done for stroller rental companies? Or does Mears want to license naming or advertising rights for their shuttle? Or does Mears want to be able to offer their shuttle as an add-on to vacation packages on the Disney website?
I think Universal skinned busses on Disney are just a fantasy.