You're in kind of a bind, since you a) can't use a false address to get auto insurance and b) no insurance company is going to underwrite you if you don't have a fixed address.
Know this, and always remember it: Insurance companies don't ask questions they can't get the real answers to. Ever. When they ask about your previous accidents and tickets, it's because they have access to that information. When they ask how far you drive to work, it's because they can find out how far you drive to work. And when they ask where you live, well... you get the idea.
If your folks really won't let you use their house as your official address (yes you need their permission, but they're your parents for Christ's sake), see if anyone else can help you out. An aunt, a granny, a good buddy, whatever. If someone says you can use their place as your legal address, you can use their place as your legal address.