Germany to Venice via Austria and Lake Garda

Hi Everybody. I remember in the old site something was mentioned about 2 or 3 things of which I am interested. I shall be travelling the second half of September from Germany (Murnau - between Munich and Garmich Partenkichen), through Austria to Italy via Lake Garda and then to Venice. Our motorhome is 30ft ( maybe 34ft if we change it). I understand that there may be extra charges for this size. My questions are:-

1. What are the possible routes and charges through Austria to Italy.
2. Which would be the better route? 1. Take the mountain pass (Brenner) via Autobahn, or (2) take the old road which runs in parrallel.
3. We have Dutch and German couples coming with us. Would it be cheaper to take another car so they can return to Germany whilst we carried on to Spain. Or, would it be cheaper to go only in our motorhome and return back to Murnau.
All your help will be most appreciated.