Tothill wrote:
I think you would find a class C up to 27 feet to be much more feasible for your trip.
I say up to 27 feet, because that is what we have (we both drive it) and it will fit in 2 parking stalls back to back.  We have parallel parked it.
We have driven it in downtown Vancouver (and the Vancouver Airport) and Calgary without any problems.  Most attractions have RV parking available.  Most cities (at least in Canada) have park and ride (easy access to local public transit).
You definitely will get more bang for your buck with a class C.
And if you find yourself near a big city and wanting a car to drive, it can be less expensive to rent a car for a week than for 4 days.
You will need to make sure the baby's car seat properly buckles into the RV or tow vehicle.
You will need to outfit your RV (whichever type you buy) with kitchen items, linens, pillows etc.  You can buy some of this second hand or there is an Ikea not far from YVR where you can buy everything you need from tea towels to a broom and dust pan.
If you purchase a BCAA membership you can arrange to have your vehicle inspected as well as have roadside assistance.  http://www.bcaa.com/membership/choosing-your-membership/compare-membership-options?gclid=CJmCkbjC9b0CFRRbfgodtHcAhQ  I know some are not impressed with AAA roadside assistance in the USA, but we had a tow on Christmas day with our RV.  It was worth every penny.
I envy you your adventure.  
The weather in Coastal BC has been warm, but wet lately.  It will be warming up soon.
Good heads up on the BCAA - I checked them out (they actually offer the vehicle inspections service to non members for $20 more) and that seems like money well spent. Our auto cover here has reciprocal agreements in North America so we essentially have AAA.
The idea of a class C motorhome is very appealing however I am concerned that on such a long trip renting cars every few days will get a little tedious and will add up in fact to thousands over 6 months.  Also we are clumsy new first time parents to a five month old and being reliant on public transport to get in and out of towns seems less appealing than just having your own vehicle.
Also, as you point out, there seems like it can be difficult to put a baby new born seat somewhere safe (airbags not being able to be disabled, shape/positioning of front seats) - though happy to hear form anyone with experience in this.