I agree with the great pyrenees suggestions. They are very good a guarding all that is theirs without overdoing it. A close friend had one that would kill any snake without flinching. It lived with about 25 barn cats that it left alone because it was taught. It covered about 160 acres which has a creek. None of the coons, armadillos, or opossums ever got near the house.
It would great you at your vehicle but as long as you were friendly it was too.