Oh and don't overplan things. Just look up national routes based on a plan you make using google maps. After doing that, look up national routes on Belgian/Dutch/German websites. In my last post you find 2 examples of routes from a website I have found. Pretty much explains how to plan a tour through Europe.
Forgot to mention a Belgian route site:
Fietsrouteplanners - Vlaanderen - Nederland here you can click on 'knots' to make a route. You can export the route as GPS and you will know exactly where to go. Awesome tool.
ADFC | Rheinradweg
You can use the Rheinradweg to cycle to south of Germany.
ADFC | Donauradweg
Donauradweg 66. Using this national route (which is non-existant to ECF), you can get all the way to the austrian border.