We have only been to Croatia, and only the Istrian peninsular at that - before it was EU. It was all perfectly straightforward. Wildcamping is not allowed, but we did talk to a Dutch chap who said he had done some wilding, usually without problems, though he ad been asked (told!) to move on one occasion by the police.
I would look at insurance issues - Serbia seems to be covered by some companies these days, but the others can be difficult - Caravanguard seems about the best option.
I would check out issues around tools/vignettes - most countries will have this if you Google some appropriate terms.
Good luck wherever you go - let us know how you get on.