I cannot speak for Western Canada destinations because I don't have RV travel experience in that part of our Country. However I would propose Banff, Lake Louise and the Rockies based on the reports of others. In Eastern Canada, it would be the a trip from Ottawa to Montreal, Quebec City through the Gaspe Pinnninsula and on to the Maritimes including Nova Scotia, Prince Edward Island and St. Andrews by the Sea, New Brunswick. If you have the time and want to spend the money on the ferry, then a trip to Newfoundland can be included.
It's more than a top 3 list, but if you are planning a trip, that's the way I woud do it.
If you need a more concentrated version of the above, give me an idea of what part of the Country you want to visit and at what time of year.