I camp in northern Greece every year, It is about an eight hour drive for me from Novi Sad.
Instead of getting the ferry from Venice, you could make the drive through Hungary/Serbia/Macedonia. Or Croatia/Serbia/Macedonia.
The main roads are good, there is good camping, and there are loads of off roading possibilities.
From Vienna you could make Greece in about 15 hours (driving time)
Also I have found that northern Greece is quite a bit cheaper than around the south or the islands.
Last summer I drove to Thassos (most northerly island) loads of campsites. And I did spot another British Landie there. (red 110 IIRC)