I have come across this description by someone who did buy a German motor home, may be of help
"In Germany, vehicles are given a new registration number every time they change hands, and also if the owner moves to a different part of the country. It seems that this process has to be done in person at the local registration office. We had to take all the documentation and the original registration plates to the Zulassungsstelle to get the registration changed into my name.
Well, we had a long wait. There was a ticket queuing system in operation, we took a numbered ticket and waited our turn. I was very grateful that Harry had come with us as my knowledge of German is almost non-existent, and I would have found it very difficult to find out what to do without his help.
The registration process seems to be done in two stages, I'm afraid I didn't follow exactly what was going on as Harry did all the talking. Basically, we were issued with a piece of paper giving the new registration number which we then took to a kiosk in the car park of the Zulassungsstelle to get the new plates made at a cost of DM50. German export plates are valid for 15 days and have an orange stripe at one end with the date of expiry stamped on them. We then took the new plates back to the Zulassungsstelle where they were validated by fixing a sticker on, and the paper work completed, the cost was DM76.
I have to say that I was not impressed with this system, I think it would be difficult to devise a less efficient one! I would also say that unless you speak good German, or have a very helpful vendor (as we did), that it would probably be advisable to buy from a dealer, who could sort out the registration for you
Once we had completed the registration we drove back to Harry's house and I handed over the cash (yes, we hadn't actually paid for the motorhome yet!) and got a receipt from Harry."