It's actually not easy at all to find actual official legal information about some of these countries.
In general, it seems that wild camping is mostly tolerated, however, technically illegal. As long as you are smart about it, you should not find yourself in any kind of trouble.
Make sure to pitch your tent out of sight, leave no trace, avoid touristic areas, etc...
You can find a complete overview of wild camping rules for every country in Europe in this article that I wrote recently: Wild camping in Europe: Complete overview. I will include the parts about the relevant countries below.
Wild camping in Bulgaria
Wild camping in Bulgaria is illegal, but mostly tolerated. That said, recently Bulgaria added a new law that explicitly forbids camping outside the camping spots. There are a couple of places on the Bulgarian Beaches where people wild camp the entire summer, I guess this was the reason that they included this new law.
I don't think they want to target hikers who only stay one night on the same place, as people still tell me it's tolerated, but legally it's not allowed. I would not advise against wild camping in Bulgaria, but make sure to stay discreet. Only make a fire if you absolutely have to and if you are completely out of sight, even then, keep it modest. Be extra careful when camping in the popular National Parks.
Wild camping in Serbia
Wild camping in Serbia is legally allowed, but not in the National Parks. But even in the National Parks, these rules are not strictly enforced. You shouldn't have any trouble with wild camping in this country.
Wild camping in Macedonia
Wild camping in Macedonia is legally not allowed, but tolerated. A lot of locals also do this. Macedonia is a very beautiful country, with a lot of places where you can wild camp without bothering anyone. I would not advise against wild camping in Macedonia, but I would encourage to be discreet.
Wild camping in Albania
Wild camping in Albania is very tolerated. I was unable to find any official restrictions, however, some individuals claimed that it is technically illegal. Even if there are restrictions, they are not strictly enforced at all.
There is a lot of wild places that are perfect for camping, you shouldn't expect any trouble. In this country, hospitality is still a thing, highlander culture is very strict about taking care of their guests. You might want to keep this in the back of your mind if you want a break from sleeping in a tent for a night.
Wild camping in Greece
Wild camping in Greece is legally not allowed, but tolerated in certain parts of the country. Make sure to stay away from the touristic places if you want to go wild camping. Sadly this includes almost all of the Greek beaches. Additionally, you should probably avoid wild camping in the National Parks.
If you find yourself in a very remote area in the middle of the mountains, you shouldn't run into any trouble.