Thanks for browsing our website and checking out one of our books! This is a tough question because it depends on where you go and who is lurking around at the time. When we have traveled around with a car full of bags we have always been very careful as to where we park and if we are unsure, we have always had one of us stay with the stuff. In 2.5 years living here and many vacations before that, we have never had our car broken into. But, we have heard stories of people who have.
Our best suggestion is try and get to your next destination and offload your luggage before going out and doing things. If you are stopping at sites along the way, sometimes there are attendants that get payed to watch the parked cars. Other times we have backed the car right up to the front of a restaurant so we could keep an eye on it. It is also a good idea to keep a separate bag with all your valuables that you can take with you. Never leave your passports, phone, laptop, etc in the car. If you come back to a broken in car, it will be a pain but at least you will still have all your important things. Hope this helps you out and you have a great trip!
Answer for Road trip safety