I've asked a friend for her comments on this. She and her partner travelled last year to Istanbul via Hungary, Romania and Bulgaria, then returned via Greece, Albania, Montenegro and Bosnia Herzegovina. She loved Montenegro in particular. She has written as follows about the return trip:
Our trip back from Greece was through Albania, Montenegro, Bosnia & Herzegovina, then back into the EU (Croatia, Slovenia, Italy, Austria, Germany, Luxembourg, Belgium, France). We unfortunately didn't have time to visit Macedonia or Serbia but would have liked to. I would very highly recommend visiting the Balkans - especially Montenegro which was by far the most beautiful, breathtaking country of the whole trip. Car insurance was not at all a problem. There are offices at every major border to the countries where you can buy a green card. This is usually a fixed price (roughly 50 euros for a month for each country). We bought it at the border and then showed it to the border control as we passed through, along with our V5C. At one border (Montenegro to Bosnia), the insurance booth was closed and the Bosnian border patrol officer was very helpful, said we can pick up a green card for Bosnia from a booth just back over in Montenegro. We just did a U-turn, explained what we needed to the Montenegrin border officer and went to go and get one. I think actually it was 7 days green card and didn't cost too much, maybe 20 euros (make sure you carry cash as most of these places don't take card). The Montenegrin border guy was totally fine - he held our passports while we went to get the green card as we explained we weren't returning to the country, just needed the shop. Passport was then duly returned and we carried on into Bosnia no problems.
If you choose to go this route, I have a few must-see recommendations!...
Montenegro - there is nothing like exploring this country in a campervan, stunning! Do not miss driving through Durmitor National Park. We were travelling from Zabljek to Pluzine and the sat nav really wanted us to go all the way down and up again on the highway. We instead followed a winding road (a quick Google Maps search suggests it's called the P14) and were rewarded with the most incredible views across the mountains. Even Gavin was forced to admit that even Scotland isn't a patch on this! We passed a couple of vans setting up for the night and wished we'd had more time to do the same, but we'd booked white water rafting near Pluzine (along the Tara River - the worlds 2nd deepest canyon after the Colorado River canyon) and had to crack on. Speaking of Pluzine, there is a great little jazz cafe/restaurant/guest house called Zvono - one of the best meals of the trip, according to Gavin. We also quite enjoyed the local lager, called Niksicko Pivo.
Just over the border by lake Shkodra on the Albanian side, there is an absolutely fantastic campsite. As you know, we mostly wild camped but my goodness if all campsites were like this one then we would camp more often! Very reasonable, very secure and safe, very international, very clean. It's right on the shores of the lake with options for tents, campers and even glamping if you fancy renting a lodge. There was a long jetty/pier walkway stretching out over the vegetation on the edge of the lake to the middle where you could jump off and swim. Utterly idyllic. There is a good restaurant on site too, with good and reasonable local cuisine. We'd have happily stayed there three more night if we could.
Albania's tourism industry is rapidly growing (perhaps too rapidly with a lack of sophistication and real understanding of what travellers may look for in a country? I missed seeing the "real" Albania but couldn't move for beach side resorts...) - it's billed as the Albania Riviera with the stunning coast but at a fraction of the cost of the equivalent in France. As such there are tons of motorhomes everywhere and so it was hard to find a decent wild camping spot by the coast. That said, the campsites or parking spaces are inexpensive. We found a beautiful spot in a field right by the sea for a measly 5 euros, in amongst the goats! I think we mostly found these places on park4night.
In Tirana I'd recommend a trip to BunkArt - this is an insanely huge underground bunker built by order of an ageing and increasingly paranoid Hoxha. It is 5 storeys deep in the heart of the capital, built to house the whole government if needed. It's now been opened to the public as a art gallery/museum. Really fascinating. There are over a quarter of a million bunkers of various sizes all over the country!!
Our favourite spot we parked, across the whole 5 months away, was at Lake Prespa, in Northern Greece. Prespa lies at the border of Greece, Albania and N Macedonia. We found the most idyllic of all spots via park4night. I feel like those people who don't want to share a good spot so it's not ruined but I couldn't be so selfish as to keep this one to myself, it's just too beautiful! If we'd had more time, we'd have liked to also explore Lake Ohrid particularly on the Macedonia side. We chose not to drive into Macedonia as it would have cost another 50 euros for the insurance and we'd only have been able to spare one day. Instead we diverted back South to drive to the Albanian cost, via the Blue Eye spring. If you have time, however, why would you not include it in your visit?
I haven't mentioned anything about Bosnia & Herzegovina. We only managed to get 2 nights there, not even enough time to swing by Sarajevo. We had such a tight schedule to get back - to put it into context, after Bosnia we went back down to Dubrovnik (couldn't go all the way there and miss that!) and then realised that we had a ferry booked from Calais in 7 days yikes! In Bosnia, we visited Mostar and then the utterly beautiful town of Pocitlj - a pedestrian town built in stone set up the hillside with paths lined with pomegranate trees and the ever present olive trees. From the castle ruins at the top you look down over the minaret of the mosque down to the river, where the sun set. We only got a couple of hours there but I can see it as clearly as if I was still there.
That's all my friend has had time to write. I hope this might encourage a few more of us to consider venturing into these countries.