Before making any decisions, I would check the money factor. I have no idea how US or Canadian dollars stack up compared to your Australian $. The Canadian $ is at an all time low against the US dollar. I would do the math with the exchange rates and see what makes the most sense. It might affect your decision to just stay in one country. Also, checking on rental of an RV, just make sure whatever you rent can be taken across the border from one country to the other. Check on drop off fees as well if you can't return it to your point of rental.
Ditto above remarks on weather in the Canadian mountains in September.
Good luck with all your planning - I think travelling with a 6 month old should be good - you won't have the worry of a wandering child at that age.
