I have left my car for up to a week while hiking. I have never had a problem.
Leave no items visible. Bears will see objects in the car and assume that there is something of value.
Parking at the Portal is quite extensive, so it's a good idea to have one location where you leave the car. That way, when you hike out, you know exactly where the car is, and you don't have to go looking.
Have a secondary option just in case the overflow is full.
paul