We have been there several times, again last summer (2015). Here are some ideas:
Going through Switzerland to Lake Garda:
The ride from Davos over the 28 via Santa Maria val Müstair over the Umbrail pass to Italy is a nice ride. TopGear listed it once as one of the best rides on this earth (we think that is overrated, though...).
There is a very nice camp site in Sta Maria val Müstair, just before the climb towards the Umbrail. We were there last summer when the Swiss SpaceCamper Club had their meet there:
When on top of the Umbrail, the Stelvio pass is just to your left. Go there if you can't resist; we think the Stelvio is highly overrated: too touristy, ugly, too busy.
Much nicer is the Gavia Pass in the Stelvio National Park (SP29 out of Bormio). Much less known, quieter, not 100.000 motor bikes and 50.000 cyclists on the same road... A very nice drive!
As a rule of thumb: don't do any of the Italian passes in a weekend if you can avoid it ... ;-) There are just too many day tourists about, then!
From there you can easily reach Lake Garda:
When on the west side of Lake Garda, don't forget to drive the SP38 from Porto up to Pieve. A beautiful drive, it has been used in many car commercials and also in the car chase in Quantum of Solace!
From Lake Garda you can take any minor roads west via Lake Idro, Lake Iseo, Bergamo if you like, to e.g. Lecco, Bellagio, Como and on to Lago Maggiore.
From Lago Maggiore, a very nice drive back is via the Simplon and Grimsel passes back to Switzerland. Nice places to camp on the Brienzer See from were you can take the old steam train up to the Rothorn (we've posted about it before):
and above the Vierwaldstätter See (Luzern), camping Hostetten in Ennetmoos:
Avery nice guide to help you along, suggesting various nice drives in Northern Italy, is the "Back Roads Northern & Central Italy" guide from Eyewitness Travel:
Have fun!