Taking the ferry from Levis to Quebec is easy and costs very little. However, I would hesitate to say that you are assured of that much parking for large vehicles in the middle of the day.
FWIW, if you've never been to Old Quebec before, it's hard to imagine just spending just a couple of hours there. There are excellent museums, lots of shopping, a walking tour, and lots of good food! We usually stay at the KOA just down the road from the ferry terminal. We never seem to have any trouble finding things to do for a day or two. You can try our red wine and French pastry diet--you're never hungry but neither do you lose weight!