I can't tell you much about sites in northern Slovenia but we caravanned for a couple of weeks at Camping Galeb at Omis, south of Split, in August 2010. In our pre-planning we found the "Alan Rogers Italy, Croatia and Slovenia" campsite guide very helpful.
We visited Camp Bled in Slovenia on the outward journey as well as stopping off at 2 German campsites each way.
Generally camp sites in Croatia tend to be VERY large. Beaches are, also, in rather short supply. Camp Galeb had one but I wonder if it was not partly man-made.
You can read my review in campsite search section
Croatia is a lovely country with friendly people and still relatively low prices. If you're planning on driving it seemed a long way to us. Our round trip from St Helens, Merseyside to Omis via Dunkerque was 3,300+ miles.
Camp Amarin, in Rovinj, about 10 miles I think from Pula, lovely site, staff all spoke English, right by the sea, with pool and slide, boat trips, have done a review. Any questions, don't hesitate to message me.
------------- Julie
Summer 2016 Camping las Dunas, Spain
Woop Woop I won £50 of Halfords vouchers for one of my camp site reviews - make sure you do yours!
A day of all things motorhome and campervan. The show is held both in the main indoor arena and across the grounds. Catering, hot drinks and food options are available. Easy access to the site and free parking included. Win one of five pairs of day tickets