I would stay at Milton Heights Campground in Milton Ontario just west of Toronto close to the 401. A very nice well managed park. Take the Guelph Line exit and go north and then take the first right and run along that for maybe 3 km and you will see the campground on your right. Entrance on Tremaine Road.
Also I totally disagree on avoiding the 407, it is wonderful and we use it whenever crossing Toronto or even going into Toronto. We always take the TT on the 407. It misses the Toronto zoo, gridlock and is a pleasure to drive. Totally bypasses Toronto with very few large trucks. It ends at Brock Road and just take that south to get back on the 401. You will not regret taking it. Well worth the toll.
You will not get into Rockwood or Darlington in July unless you get really lucky making a reservation 6 months ahead and you have already missed that window.