It's a real problem as whilst there are many campsites in the Netherlands, in Germany they become far fewer the further you move from the coast.
The bike line books give camp sites on their maps and for Radweg 1 to Berlin and onwards you will see that in the old East Germany part of the country, specifically Harz, they are uncommon.
What I do, and I'm sure there is a better way, is to carefully plan ahead the towns along the route about 40-50 miles apart and then emailing the town tourist office or by looking at their web-site for campsites. A print-off of the maps or a scan to your tablet gives you a good idea of where they are along the route and what they are like. To get to Berlin which is about a 1000k would take me up to ten days so maybe about 6 /7 nights in Germany to sort out.
Using the Bikeline book along R1 to Berlin:- From Arnhem going East there are campsites not far from Munster, Warendorf, Schloss Holte-Stukenbrock,Detmold (I was camping there a month ago), Hoxter, Holzminden, Bad Gandersheim, Langelsheim & Wolfshagen,Bad Harzburg.
Now you are in the Harz mountains so a wild and illegal camp might be necessary ( but people do it)
Next place as far as I can see with a camp-site is Bernburg and then on from Dessau, Wittenburg and Potsdam there is no problem.
If you do decide to go the R1 route, I suggest the AFDC route map or the one published by PublicPress. I've used both and they are spot-on for accuracy and easy to read. They are in German but still will be a great aid as they are full of useful info, especially the AFDC . PublicPress maps are in concertina form, are a laminated paper and so easier to use on the bike, and are cheaper but have less tourist info but very good, I like them. http://www.publicpress.de
Best of luck, I've fancied doing R1 through to St Petersburg but don't have three months
There are many excellent campsites at Potsdam and campsites at Oranienbaum, Dessau