See the "stickies" at the top of this "RVing in Canada and Alaska" forum for fuel stops and camping suggestions.
And, if you are planning to use your cellphone in Canada, it can be very costly unless you pay extra for a "Canada plan." Just remember, you won't have cell coverage on a lot of the highways you will be traveling, so calling ahead for reservations is not possible from many areas.
We didn't dry camp at Walmart in Alaska, but we did in the US on the way up and in Hinton, Alberta, on the Yellowhead Highway just outside Jasper National Park. But dozens of campers were parked at the Walmart in Whitehorse, some even sitting out in their lawn chairs under their awnings!
And, as the others have said, you can pull off the road anywhere that it is not posted. One free campground we used was just over the Alaska border from the Yukon in Tetlin National Wildlife Refuge. We had the closest site to the lake, with lots of trees but no potable water and no dump. So, if you stay there, be sure to bring plenty of water and plan to dump in Tok. We dumped free (with fill-up) at a gas station in Tok and also got a free high-pressure truck & RV wash at the same station at the same time. The wash was really needed after traveling through several long highway construction zones between Destruction Bay and the Alaska border.
Have a great trip!