Roermond it is.
Though the purists on this site might want to remind you Yin Yang is a Sauna Club, not a FKK.
For real FKKs you'd have to be heading towards Dusseldorf, Koln or Frankfurt.
Here's a map that might help -
http://www.isaw-db.com/map_de.html
Once you've got names of a few places, you can search this forum for reports (though a lot of the places on the map have never been reported here, and some have probably gone away).