We live quite close to Fussen, so if you decide to take that route and stay there we can give you some recommendations about stuff to do and see, and a couple of ideas for places to wild camp if that’s your thing. I would avoid going through Switzerland to cross the alps, you will need to buy a vignette for a year and it costs about €33.
Very close to Fussen is the famous Bavarian castle, Schloss Neuschwanstein, which is definitely worth a visit if you have never seen it. The lakes close by are really nice in the summer Forggensee is a favourite of ours for days trips, there are some campsites there too, but we have never stayed there because it’s so close to home.
We have driven the route from Fussen to Lake Garda a couple of times and it’s definitely possible in 1 go, I think about 6 hours with stops for tolls, toilets etc. But you could consider stopping at Kalterersee rather than Innsbruck; it’s a wine area in Italy (close to Bolzano), there are a couple of nice campsites on the side of the lake. We have stayed there and we really like it. If you like walking or cycling there are loads of great routes nearby as its surrounded by mountains.
My tips would be:
• Remember to buy the Austria vignette for the Autobahns, and buy it as soon as possible, the petrol stations selling them get busier and busier the closer to the border you get. Also make sure you check that the dates are correct on it, it’s not uncommon for there to be “mistakes”. Or make a route that keeps you off the Austria Autobahns then you don’t need one.
• Make sure you fill the tank before you cross the border from Austria to Italy, you can expect to pay €0.30-€0.40 per liter more at an Italian Autostrada rest station. We normally fill up at near home because once you get into the Alps the price increases by a few cents a liter.
• The Italian service stations are always a complete rip off, so drive past unless you really need something.
• Remember that most things aren’t open on a Sunday in Germany and Austria, and lots of places close at lunch time on a Saturday.
Hope that helps