I should really have included Slovenia in the list of countries you might pass through.
Near the capital Ljubljana (15km to the north-west of), Camp Smlednik at Dragocajna - http://en.dm-campsmlednik.si/
- has part of the site reserved for naturists.
And to the east of the country, Kamp Soncni gaj, which is part of the Terme Banovci at Maribor - http://www.ntz-nta.si/en/content.asp?id=4108
- has a large naturist section. According to the website, "The health resort's SonÄ¨ni gaj ("Sunny Grove") campground offers 130 regular camping places and 70 camping places for nudists. This nudist camp has a long history, as it was the first such camp in Europe set up beside a thermal spring. Separate from the regular campground, it has its own showers, washrooms, lockers, and a circular, 15-meter diameter swimming pool filled with thermal spring water."
Finally, I ought to have mentioned some of the best naturist facilities in Europe, 36 km north-west of Munich (next to the airport), at Therme Erding - http://www.therme-erding.de/
It is a huge textile leisure centre, but the Sauna Paradise is dedicated to nude relaxation and pampering. With a host of indoor and outdoor pools to be enjoyed as nature intended, it's much more than simply a sauna. There are three different themes within the bare area: Roman, Finnish and Tropical.