We've done Ancona to Igoumenitsa and Ancona to Patras, (and the returns) depends where you want to go in Greece. Your best bet is "Camping on board", you sleep in your van but have all the ships facilities and its cheaper than booking a cabin. If you choose that option as soon as you are parked on the ship you need to make sure you get an EHU, this will involve lots of extension cables off a few ship's cables. Ideally you also want to be on the outside of the ship where you can get your chairs out in the alcoves and enjoy the voyage. Usually a calm run down the Adriatic and the food on board is usually ok too, you should get a voucher when you check in giving you a discount voucher for the restaurant.
Having said that you are going to find it very, very hot in Greece in July/August, especially in a tin box on wheels. We've been to Greece for the last three years in springtime and its Motorhome heaven. Have a look at the threads on here started by rotiontour - he's a mine of information on Greece. pm me if I can give you any more info
Enjoy your trip,
Pat