Hi Julian.
Off to Croatia, eh? Looks a top cruising ground for small boats. Not been there myself yet as sort of locked in with friends in the Ionian but its on my ToDo list ( along with the Gulf of Finland when I  can afford the thermal underwear.)
First time I trailered abroad I had a bad case of paranoia and the following list of docs– plus  VAT receipts for everything newly bought.

1.   Boat Insurance.  Specific for the areas sailed.
2.   IYT Day Skipper Certificate and log book.
3.   IYT ICC  (Coastal)
4.   RYA National Powerboat Certificate Level 2
5.   SSR for “Four Sisters” plus red ensign.
6.   Ships Radio License
7.   RYA Marine Radio Operators Certificate.
8.   Car insurance, Green card and International breakdown cover.
9.   Cars’ Log book and MOT Cert.
10.   Personal travel insurance.
11.   NHS EHIC Card.
12.   International Drivers Licence. (Not needed in EU with the new “card” style  licenses.)
13.    Oh, Yes! Passport.

After nearly 10,000 miles of trailing and sailing in France, Italy and Greece (Do we count Wales as abroad these days?)in three years I ve only had the following incidents.
1.French Authorities - none. Unless you count the Gendarme in a RIB at the Golfe du Morbihan who assured me that the 7 knot tide had finished running!
2.Italy.
a/ Stood over by a grumpy young cop while I tied on a long vehicle sign. (RAC didnt tell me it was a requirement.)Could have had an on the spot fine for that one.
b/ Quizzed by heavily armed police on the Turin/Bologna road. Turned out that they liked the look of the boat and wanted to know where I got it from.
3. Greece.
a/ Port police in Gaios left a message for me to report with Ships Documents. Just gave a brief glance at the SSR.
b/ Cabin searched by soldier armed with automatic rifle at ferry port in Igoumenitsa. Looking for Albanian youths who d been trying to stow away on ferries. 
4. UK. (Dover-Dunkirk)Cabin searched for drugs by a very nice Spaniel. Took an embarrassing interest in old boat shoe.

Overnight stops.
In France some of the campsites cost as much as the cheap hotels. Then you ve got to find them. They re in pretty spots miles from the motorways. (People have had problems camping at Aires, the En route stop-overs.) We stick to toll roads and Premier Classe/Campanile chain hotels - a habit developed pre SatNav!

In Italy found it hard to find campsites but hotels are good (and the food at service stations is much better than France!) If you go to Croatia via Ancona I can recommend  Jolly Hotel Miramare (seriously!) for pre war elegance at Campaniles prices.

In Greece. Sleep on the beach! No one gives a damn and the mosquitoes will love you to bits.

If you need hard facts rather than my rambling reminiscences get in touch with the RAC or AA....but watch the small print if you buy travel insurance from them.