I was in Greece 1 1/2 years ago, and don't recall seeing RV's at all, certainly nothing very large.
Outlying roads are relatively narrow, the ones I was on, so I'm not sure navigating anything larger than a Class B would be feasible.
I would get online to Greece tourism and see what they say, and if they have any kind of campgrounds.
There is still a serious humanitarian crisis going on over there, with tens of thousands of refugees, so that is also something to consider....I would think their resources are pretty well focused and allocated
That said, it is a beautiful country, the people are very friendly and welcoming.
Maggie