Hi
We went to Croatia and Slovenia in Summer 2010. We did NOT have a towbar or towrope but we did have GB plates. But then we, always GB plates on the car and c'van. Cary forgot to ask for a euro plate on caravan when she went to buy it and I think it is good to have them on the car and c'van anyway. I think they add to safety as outside the UK local drivers are alerted to the fact that we are "ignorant" foreigners!!! We, always carry 2 warning triangles when towing both because some EU countries require this and because it, again seems a sensible thing to do. The same with flourescent jackets on the back of our seats to be readily available in the case of breakdown.
Remember to check that your car and caravan insurance extend to Croatia as it is not a EU country. If you plan to cross the coastal strip of Bosnia H which crosses southern Croatia you will need insurance cover for that too.
Croatia is the only time that we have had our passports stamped at a border crossing in Europe - quite a novelty!! I nearly failed to stop as I had forgotten it was different to other Continental borders.
I know that I have posted details before but have you read my 5 part Croatia/Slovenia Holiday Journals here:
http://www.caravantalk.co.uk/caravan-holidays/journals/europe
and seen my extra photos here, the sub-folders relate to the Holiday Journal Parts:
https://www.dropbox.com/gallery/63718686/1/Croatia%20%26%20Slovenia?h=84df2e
Croatia and Slovenia are lovely countries but as I always warn folk a long drive!!!
I hope you have a lovely time.
Best wishes
Tim