Good Luck!
I have done it a few times. If you do it without stopping then 4 days is about the fastest you can make the journey.
Some serious advice though... Pre-book your ferry 2 or 3 days before you get to italy (assuming you are going that way) The easiest to get to is Ancona. If you don't pre-book, you will have to fight your way through 100's of people trying to get a ticket, also there are a lot of 'illegals' trying to get out of italy. Don't be persuaded to give anyone a 'lift' and make sure you keep your car locked whenever you are away from it, and go back to check it regularly.
This time of year the ferry from Ancona to Patras is very busy. You can wait 2 or 3 sailings before you manage to get on - especially with a vehicle.
Getting from Piraeus to Rhodes is a lot easier, there are probably less people trying to get on the boat. If at all possible, avoid G&A ferries. The boats are a 'bit' old and could do with a serious overhall.
One other note on Italy.... don't stop - anywhere! not on the motorways anyway - even for petrol. If you absolutely have to! keep your car locked, don't get drawn into conversations with people trying to 'sell' you something, you will end up being ripped off to the tune of whatever you happen to have with you. - believe me, I know many people who have been conned this way. And driving a foreign vehicle is almost like putting a big neon sign on top saying 'rip me off'
but... it's a great drive
sub