I would personally just follow the route that Google Maps suggests: maps.google.com/maps?f=q&source=s_q&hl=en&q=Rotterdam,+The+Netherlands+to+vrsar,+croatia&sll=48.163552,11.58386&sspn=0.005625,0.013883&ie=UTF8&t=h&z=5&saddr=Rotterdam,+The+Netherlands&daddr=vrsar,+croatia
It's all motorways. And I am not an expert in caravans, but this route is followed by many Dutch people with caravans, I guess they just stop in rest areas and make overnight stops. I mean, in summer those routes are 24 hours full, and the rest areas are therefore safe, in my opinion. At least in Germany, which is what I know.