I can only tell you about places within a days ride from home unfortunately...since I've only been inland in the Balkans...and don't hang around on the coast much in summer.
If it's peace and quiet you're after...then anywhere in Cres island matches your description. Camping Baldarin on the south tip is nice, it's a short ride away from a decent sized town (across the causeway on Losinj island).
The other place I like isn't a campsite at all, wild camping is tolerated near to Porec here
There are absolutely no facilities there at all, not even water..although there's a food stand nearby.
Also in Istra, a little further south , there are some very quiet campsites near to Bale, here
As I said, beach camping in summer isn't quite my thing, and I prefer the seaside in winter...I'm sure other people can suggest nice places further south. It's certainly not all a party destination, and there are plenty of peaceful places.