The website Cost2Drive is a fuel calculator that works in Canada or the U.S., and taking into account the make and model of your car and pump prices, calculates the cost of a trip. Here's an example drive from Toronto to Tobermory, ON - It's surprisingly difficult to get the default routing to stay in Canada!
http://beta.costtodrive.com/from/Toronto-ON-Canada/to/Tobermory-ON-Canada/1/custom/10/12/Regular/. One of the things I love is a comparison of the cost to fly. Keep it mind, when you discover that Toronto to Vancouver in a RV comes out to $903.
The list of RVs is fairly scarce, however, so my 1986 Toyota Coachman isn't listed. As such, I need to click on the "Can't find your car" and enter its pathetic, if typical mileage - namely 10mpg.
In general a Class C RV can expect 10 - 12 mpg. Class A's (the big busses) are in the 8 - 10 mpg range. Fifth Wheels are completely dependent on the truck, but if you're getting 15mpg, praise the Lord! Ditto for Class B vans.
I typically plan my costs at 10mpg (or 3 miles per $), and then celebrate that I was a little bit harsher in my calculations than reality.
Be aware that older RV's in particular don't take kindly to the mountains, so Banff will suck down the gas more than Manitoba.
Finally, remember that gas in Canada is pretty expensive. Back in 2010, when I drove from Northern Virginia to Tobermory, ON, I found gas prices creep steadily upwards as I went further north. At the time, gas was in the low $3's when I left DC, mid $3's in PA (and the mountains were terrifying as I would floor the gas and still manage no more than 20 mph!), high $3's in NY, and about $4.25/gallon in Canada. As a Kiwi, I know you are used to buying gas in liters (which Canada does), but for us 'Muricans, remember that you have to multiply by (roughly) 4. You know you're going to be in trouble when buying gas by the quart.