The legal situation is unfortunately a bit more complicated than what is stated by Dirty-flow and user24582.
Even if the German traffic regulations do not directly forbid you to sleep in your car, you may easily violate other regulations doing so. Roads and public parking spaces may in general only be used for traffical purposes and even though stopping for a night to rest or even sleep is considered acceptable, setting up a 'home' for several nights in a mini-van is not what road-side parking was intended for. To be more abstract, it is generally considered acceptable to stop and sleep in your car if the purpose is to relax and regain fitness to continue a ride.
user24582 is right, that the Munich city council writes on their web pages that sleeping in a caravan is forbidden, but they actually have no legal means to prevent you from doing so. This article (unfortunately only in German) addresses exactly this topic and explains that when asked on the phone, the city council also confirms that they actually mean that it is not allowed to stay and live somewhere for several days. Stopping for a night of sleep is also in Munich fully acceptable.
Even if you could get around the law by driving to a new parking space each day and only stay for one night in one location, you should also consider other practical problems. When parking anywhere, you will probably not have a toilet nearby and using the next house corner or a tree in a park as an emergency solution raises new legal issues. Before someone claims that there is no German law forbidding you to urinate in public, the public leak is usually fined according to § 118 OWiG as a 'crude, offensive action'.