For Mull, which is not so big, they use the Nissan LeafI'm not sure I understand this concept.
if they need to visit family on the mainland, they leave it at the port plugged into the docking station
go across as foot passengers then pick up a cheap hire car in Oban
run that around the highlands, and down South then return it to Oban and back on the ferry
saves buying a petrol car, and it saves a fair few quid on the ferry crossing