we went through serbia/yugoslavia a week or so ago, the carnet was not neeeded to be stamoe in the border, they just stamped the passports, but we did need a valid green card, and as ours wasn't we had to pay 70 euros for one. they just take your passport from you (the driver) you park up buy one from a big long line of green card shops in lots of different languages, and give one of the receipts to the police who promptly give your passport back.
even though the toll roads through serbia are as much as fuel will be, we could only get as far as belgrade avoiding them before getting lost in the hills.
camping is fine if you find somewhere out of the way, but expect lots of enquiring locals. we were only there for one night.
don't know about campsites.
allthough a bit boring the main central toll road is the best way and fastest.
belgrade, is a little mad for driveing in but no worse than london.
hope this helps. - stuck in cairo at the moment. sudanese visas.