You might want to look at Roadtrek Class B's. They do have floorplans with 4 seats to accommodate your child car seats.
Sorry I cannot offer advice of where to purchase but 2 things come to mind. What do you plan to do with whatever you purchase after your trip. Also, you mention travel from March to August. Do you realize that March is still winter in Canada and campgrounds will not be open. In March Canadians are still going south in search of warm weather. South would be to the United States, usually to Florida or perhaps South Carolina. I'm thinking your family in Toronto could help you plan your route.
Hopefully others on this forum in the Toronto area can give you some leads or know of a suitable unit that might be for sale.
2014 Triple E Regency GT24MB (Murphy Bed) with all the good stuff
towing a 2016 Jeep Cherokee TrailHawk
Berkley, the amazing camping cat missed dearly (1996-2012)