over 3500 kg

I'm looking at going to Austria then possibly onto Croatia in September, my van is over the 3500kg, so looks like I would have to get a go box to drive in Austria, do they do it by wieght or hieght as someone told me it's the hieght of the van that registers, I only have a single axle and it looks like any normal motorhome, (Rapido 986F,) just that it was uprated when new to 3875 kg.
Any help would be apprieciated.

Richard