We have been lucky enough to have spent the last 5 years doing similar length trips each summer while still working. The longest being 2012 for six months in one go.
Its a good length of time for a single trip to really see some of the best of Europe. On our first adventure in 2009 we didnt plan very well and ping ponged around Europe at way too fast a pace and in no real logical order.
People say "Oh we never plan and just turn left at Calais". Ok if that suits you but I can honestly say since that first trip I have researched and planned much more with each year and now with the Internet being more readily available abroad our trips have just got better and better. I would recommend you research and plan where your going and where your going to stay as much as possible and dont rush on too quick when you get there. We now stay in one place until we feel we have had enough. On our first trip we were still feeling like we were on a two week holiday and rushed around too much. 16 weeks is a lot of time.
I would say over the past 5 years of touring my highlights would be in no particular order
France
Brittany
Most of the south but definately not the coast. Cut a line from the Southern French Alps across the Verdon Gorges, Lac St Croix and east through Provence, Tarn, Languedoc, Aveyron, Lot and Dordogne and you cannot go wrong.
Germany
Black Forest and Bavaria. Mossel Valley is ok but gets a bit samey
Switzerland
Lucern and the Bernese Oberland, Eiger and Jungfrau (north of and around Interlaken) are just breath taking but try and do it in ACSI season as wilding isnt as easy, not many Aires and sites outside the cheap season expensive
Italy
The Italian Lakes and the Dolomites are stunning. Maggiore and Orta are my Favourites with Garda being my least favourite only because everyone goes there.
Austria
The Austrian lakes east and south of Saltzberg are out of this world and even in July / August not that busy as everyone is under the impression its expensive. Well its not and wilding and Stellplatz do exist.
Slovenia
Triglav National park, Lake Bled and Lake Bohinj are stunning. Wilding not allowed in the park and only one or two aires
I tend to plan a general idea for the trip and a route plan. Then I will spent hours if not days on
CAMPINGCAR-INFOS which if you use google Chrome through this link
accueil it will translate into English. I also have the offline French version on the laptop. I look at all the wilding and Aire opportunities around each area I plan to visit and save them in folders as PDF files to call up later when we are nearby. Mrs D tends to research the what to see and do lists (This is all before we go).
The plan will inevitably change a bit as places are dropped or new ones found enroute.
The other important thing is to make sure you can get online whilst away. This can be useful for asking questions about new places or general local research.
Most of our trips are on here
www.hankthetank.co.uk but the last couple of trips I never finished the blogs.