Slovenia to Croatia, just showed passports.
Croatia to Bosnia, passports and 'passport for moto', which I correctly assumed was the v5. Didn't get asked to show insurance, which was handy....
The bit of Bosnia between the two bits of Croatia next to the sea, been waved straight through, and another time stopped to show passport and v5.
Croatia to Montenegro, Passport, v5 and proof of insurance. Got away with a 10euro bribe for not having insurance.
Montenegro back to Croatia, passport and v5. The border crossing nearest the coast can have a tailback a few miles long in either direction, no one seems to mind bikes riding straight to the front.
Hungary to Serbia, just Passports.
Serbia to Montenegro, just passports.
I think it depends on the kind of day the border guard is having, we've maybe been lucky.