Word of caution in Seattle: Do NOT leave valuables in your vehicle. They have a HUGE break-in problem.
We started camping with my son when he was 8 months old, in a tent. Yes, it was difficult due to the amount of extra gear that we had to take and trying to keep him on a schedule, but not undoable. Also keep in mind you will be in bear country, which presents it's own set of issues. Be extremely flexible-there are going to be times when you can't do anything because of the baby's schedule. September may actually be preferable to the warmer weather as a baby that age is unable to sweat and may be more comfortable if it's cooler.
Your schedule, I think, is too aggressive. That's a lot of driving. Having been to all of those places except Portland you would not be doing them justice by spending only a few days in each. Here's what I personally would do:
I would do Banff, Jasper (4.5 hours north of Banff), Yellowstone and Grand Teton (possibly add Glacier/Waterton, flying into Calgary). If you choose to fly into the states, go to Idaho Falls and not Jackson, WY. It's generally less expensive and the trip through Idaho to Montana is stunning.
OR
Vancouver, Victoria (fantastic city, one of our favorites) Seattle, Olympic NP and Portland. We did Vancouver, Victoria, Seattle and Olympic in one two-week trip. Portland is not far from Seattle-about 3 hours. You could possibly add another spot such as one of the other national parks in WA State.
Either of the above itineraries would allow you to slow down, spend less time in the RV, and allow for changes in plans if there are any issues, baby-related or otherwise. That time of year you can "plan on the fly" as the parks and campgrounds are much less crowded so reservations are not an absolute necessity except on Labor Day weekend as stated above.
Where to rent an RV will be based on how strong your currency is against the US dollar or Canadian dollar. Right now the CAD is trading on almost a par with the AUD but the AUD is only worth about 70 US cents. At this point in time renting in Canada is more economical than in the US but it's hard to say what may be the case in September.
Your decision should be based not only on economics and ease of travel, but ability to travel. One child now, two kids 3 years from now, etc. If this might be your only chance, go for it. Even if it isn't and you are willing to take the additional responsibilities and inconveniences of traveling with a young child then I would go.
I may not have gone where I intended to go, but I think I have ended up where I needed to be
Douglas Adams
[purple]RV-less for now but our spirits are still on the open road. [/purple]