There's plenty of camp sites along the coast of Croatia, some through the centre of Slovenia....ditto Romania - but you won't find many in Bulgaria apart from on the coast (if you get that far) - the one's in-land leave quite a bit to be desired!
Camp sites I stayed in last year included,
- Bran in Romania - the castle is theoretically the one upon which Dracula is based but there is some pretty countryside around and Tony is great especially if you have problems (like we did!) , in Romania we also stayed in Carta, Cluj Napoca and Sibiu.....the alan rogers website has a range of good quality campsites.
Wild camping is part of the social scene in Romania and accepted - but if you go into Bulgaria, the same is not true so take care!
If you go into Croatia, if you get time, do try to get to the Plitvice Park
- they are stunning. I know some people have mentioned "mine field" signs - if you go off the beaten track between Plitvice and the coast, you may well come along a single track road with mine signs on both sides - we certainly did last September.......not a problem as long as you stay on the track and the scenery is certainly worth it. There's a couple of campsites near to Plitvice - and one does have cabins (but you need to prebook).....alternatively, you can get a room pretty cheaply (I think ours was about £20 for one night for two) ....in low season you can pick and choose and get a nice one!
Also in Croatia, I'd recommend the ampitheatre in Pula
.....I never did do the collaseum in Rome, but this was pretty special...not over-run with tourists, quite charming and with parking right outside!....plus there's a decent campsite
about two miles up the road who does a good line in sunsets! (I think it was Stoja - but don't expect much from the on-site supermarket!)
Favourite place in Slovenia was Lake Bohinj and the campsite
about 6k's up the road was quite decent, as long as you go in good weather (the shower/toilet block is cold otherwise).
Hope this all helps!