Hey, I was biking in Italy toward the end of July last summer. Unfortunately, because of priority differences with my biking partner, we didn't have a chance to do too much cycling. However, there are a few words I can pass on.
Plan for a HOT trip in July - temperatures can soar. This is great at night, especially when you're camping, but it can be draining for biking during the day. Hydration is key to survival, literally. Early morning and late vening biking is the best bet, with a few hours of siesta in midday.
Italian roads aren't the most bike-friendly in Europe, but you may be able to find a good route with enough research. Remember, this is when all the mad motorists from Northern Europe come down in their sports cars and motorcycles -- you'll probably be trying to balance between road quality and motor vehicle volume when you're choosing your route.
If you want to experience Italy on a low budget and don't mind missing the big cities, I'd recommend heading right to the southern boot. Much less trafficked, and the whispers say that this area is much like Tuscany region was.... 20 years ago before tourism.
If you do plan on going to Venice... DON'T TAKE THE BIKE! There are camping grounds on the mainland, leave your bike there. Access to Venice is via a single highway bridge, and it's as much as your life is worth to cycle along that, no matter how experienced you are. Vencie was not designed with bikes in mind.
If you plan on taking trains, be aware that catering to bikes isn't stellar. Your best bet is regional trains, but regional train travel is slow -- book at least a full day, if not two, to get across the Southern boot of Italy (from Naples to Bari for example). Regional train travel can get HUGELY crowded on summer weekends. Unless you want to have the entire, overfilled train car get off the train when you're offloading your bike, plan around this.
That being said, if you want to compromise with the gf, take some regional trains to a promising biking area, and spend some time cycling around there. You can free camp, or ask permission of farmers. At worst, let her lounge on a nice beach while you get your touring in

If you only want to skirt Northern Italy, plan a couple of days to visit Cinque Terra. That's all I'm saying - only those who are willing to do the research deserve to find this place.
Camping: very difficult to get spaces in more quality campgrounds in southern italy during summer weekends. Usually, charm will get you something if you don't book, but it may only be a space beside the parking lot. Wild camping near a beach is probably the best bet - hygiene is easy when you've got an ocean nearby!
If you want info on hostels (during much-needed hygiene breaks), try
www.bugeurope.co.uk
Italy's nothing like the U.S. or New Zealand. Tips for convincing your girl: The south is incredibly cheap, wonderfully friendly, and very warm. The food is the best you'll taste in all three places, and you can eat like a king on very little. Fresh ingredients like nothing you've ever tasted. Gelato - Need I say more? It's the staple of a biker's diet. A caring atmosphere, and very mediterranean appreciation of life can be found in the smaller, less touristed towns. People aren't as structured by rules, they are more spontaneous than our time-oriented counterparts in America or New Zealand. You'll regret missing Italy, but make sure you plan well to avoid the pitfall of a hot, summer toursit season.
Best of luck to you!