Germany, Austria, Hungary then thru Serbia I have done and can vouch that the motorways are good all the way. In a motorhome you should not expect any border difficulties leaving EU into Serbia… Then at Nis in Serbia split for either Macedonia or Bulgaria depending where in Greece you are headed. Obviously you need to be prepared for winter weather all along the route but Hungary and Serbia are largely flat. I advise against the Croatian coastal route at that time of year as in inevitably leads to mountain passes into Kosovo, Serbia or Albania