We have done that a couple of times and definitely take the tour of the glacier on the ice bus on the way to Jasper. I remember when we were in Tunnel Mtn. CG when my daughter & I were playing cribbage and noticed movement outside our window. When we checked there were 5 elk standing there watching us play cards.
Canada is great for the type of trip you are planning. RV rentals will include everything you need but clothes, food, and toiletries. In May it can be cool so you will want to take extra clothes and pay the airline for the extra luggage.
When you check out the RV be sure that the LPG tanks are full and the gray and black water tanks are empty. Most outfits in the past turn over their RV inventory frequently so that the rigs are always under warranty and this gives you a great chance of having no problems on your trip.
President's Choice stores are hypermarkets - think of a Costco, Trader Joe's, and high quality supermarket all rolled into one. Nothing like it in the USA. One stop there and you can get everything you need in terms of food and other items for trip and fill in with stops to the government run liquor stores if you want to have wine.
I have never relied on GPS in Canada but if you have a unit take it along as it is likely to be for North America and be adequate for your trip.