June is pretty full of family obligations chez Pratt and August is both hot and high tourist season, so September is my default. Last summer in Quebec, I was always able to find a campsite, but the campgrounds were all near capacity with permanent camps. People seemed to be using their RVs for the R function, and writing off the V function. They were like little cottages.
One idea I'm toying with is to ride to Montreal and take a train to Moncton or Pictou and take the shuttle or ferry to PEI, or IPE, if you prefer.