Where can you camp wild in Germany?

In Germany, wild camping is generally prohibited. In North Rhine-Westphalia this is regulated by the State Forest Act § 3, in Hessen by the Forest Act. Camping and parking caravans and motor vehicles in the forest is prohibited and requires permission from the authorities or the landowner. 

Where can you stay overnight in Germany with a motorhome?

You can stay overnight on designated sites, and campsites also have sites for motorhomes.