From what I understand of the LEZ zones in Germany, they're really only applicable to some big cities and so generally driving around on the highways or small towns/villages is no issue - though I'm not an expert, thats just my understanding. We have though managed to get a green (appropriately!
) badge for our T4 because it has a catalytic converter, so we're good to go into the cities too...
In Brussels for example (where we live), the LEZ laws are only just coming into action and actually before re-checking today I was under the impression that motor-homes/campers were exempt, but upon re-checking, seems they've changed their minds and we will now have to apply for exemption - oh great! That will be fun! We only live 2km within the LEZ perimeter...
Regarding foreign vehicles, for example in the Brussels LEZ, currenty no issue, but seems towards the end of this year you will have to register your vehicle before entering the zone. Sounds to me like a very well thought of process (please note my sense of sarcasm here...) - and like Hotel says, typically each city seems to have its own rules - anyway nothing to see here, continue round the Brussels ring and head south
So for example, the 'current' (meaning check back again as it will no doubt change!) Brussels rules:
http://www.lez.brussels/en/content/my-vehicle
Found this site which has info for other European cities:
http://urbanaccessregulations.eu/userhome/map