Harry Monk:
I would transit Hungary, then either Romania or Serbia, and then through Bulgaria into Greece,
According to my friends, I would not advice going via Romania. My friend used to travel regurarly between WrocÅ‚aw, PL and Asenovgrad, BG, and when I was going Turkey in 2006, he told me “If you like hazard, go via Romania, if you want to pay certain amount of money, go via Serbia”. According to him, in Romania cops and other uniformed forces are always seeking for bribe, or they create some crazy rules like “extra insurance” “car disinfection” “bridge toll”, “border toll” etc.
In Serbia you have to pay for motorways (which are sometimes single carriageways 
) and it was about 27 Euro for a car three years ago. In Romania it’s all depends of how many police car will spot your foreign license.
My other friends were in Romania last summer, and it seems that not much changed - they were stopped by police car, they took all documents off them and told them that they have to pay to have it back. They were really desperate to do not pay, but after 6 hours on the hard shoulder (!) next to police car, they gave up…Next three bribes they paid on the spot, as it was much faster and much cheaper.
At least in Bulgaria when you pay a bribe, they give you a receipt! (I am serious!!!) and you don’t have to pay again on that day 
If you are going to Eastern Greece, I really recomend you to see the Rodops Mountains.
And no, you don’t have to buy any extra insurance for Serbia if you have green card.
Also: south of Budapest you can save money by going off the motorway - there is really good road along, with not much villages or towns on it and few nice places to see… And on motorhome it should not be much slower, especially, that lorries are banned from this roads, unless they have some delivery there.