Some friends and I road-tripped from Ljubljana to Athens in 2011 and we wild-camped the entire way. Albania you can really camp anywhere, Croatia you should be a little more careful, and I'd say Slovenia is in between. We didn't camp in Montenegro but I think it seemed pretty relaxed, similar to Albania.
In Croatia it was probably the hardest, cos we were often staying close to national parks, but if you pick a secluded enough spot or even just somewhere that is fairly well obscured from view you should be sweet.
In one case in Croatia we camped only 20 minutes drive from the carpark for Plitvice lakes national park, in a tiny clearing in the trees between the main road and someone's farm. When we came out in the morning we realized we'd only just been out of sight of both, and had probably been lucky to get away with it, especially since we lit a fire and cooked.
In another case in the south of Croatia where it was less built up we stayed on the edge of a field, and woke up to the farmer plowing with a tractor not 50 meters from us - he seemed unfazed.
Albania was the most relaxed, and you can wild camp pretty much anywhere. We camped in a farmer's field one night, and were lucky enough to be able to camp less than 100 meters from Siri Kalter springs another night despite there being no camping there. The caretaker who we thought was coming to kick us out, and who spoke only Albanian, was only concerned with whether we had a tent to put up somewhere!