Badhabit:
In the PEI you will have a hard time finding an open campground after labour day. The KOA in Cornwall may be one of your few choices. Maybe the National Parks??
In New Brunswick and Nova Scotia most campgrounds are open until Sept 30th and many until Thanksgiving weekend. No problem finding a site until mid-October but after Thanksgiving your choices will very limited and in some cases you might find an open campground but the water service may not be available (freezing weather)
Letting the snow chase you south really will not work. No snow (normally) until late November however there will be many below freezing nights, and I mean a hard freeze, before the snow starts. In November we can get a lot of nasty freezing rain and driving condition can really suck.
Most south bound travellers that want to travel based on signs from Mother Nature tend to follow the fall colours south. Late September here, down through New England in early October, Mid-Atlantic states late October and in warm weather for November. These same travellers often follow the apple blossums north in the spring for the return leg of the trip.
Your trip sounds like a blast with a good mix of planned stops and some seat of the pants exploring as well. Just plan to be headed south from here by mid-October and you should be fine.
Safe Travels