Coming north from Denver via Montana into Alberta, you'll find Walmarts in most of the major cities, between Lethbridge and Edmonton. It thins out as you head north towards Dawson Creek. That said, there's a Walmart in Dawson Creek at the start of the Alaska Hwy, and another one in Whitehorse YT, and that's about it until you hit Fairbanks. There are also a smattering of truck stops along that route - check the truck stop websites for specific locations, but assume major cities have them. IIRC, there's a Flying J in Whitecourt, AB, and that's the last full service, big 3 truck stop (TA/Petro, Pilot/FlyingJ, Loves) up there. There is a Pilot/FlyingJ listed in Dawson Creek, but it's just a cardlock fuel station. No facilities. As others have said, if you want true rustic boondocking, there are plenty of places to pull over along the Alaska Hwy/Hwy97 route. Many are just gravel pull outs, like min-rest areas with gravel parking some have pit toilets or porta-toilets. There are also small government campgrounds in northern BC and the Yukon with iron rangers for paying the C$12 to C$20 per night for a place to park. Most have toilets, but not much in the way of other facilities.If you're creative, you won't have any problems finding low cost accommodations. Just use common sense and you'll be OK.
As an after thought, there's a campground that's about $16/night with showers, dump station, but no hookups in Tok, called the Alaskan Stove Campground. It was self serve payment late in the day. We stayed there and it was fine, but it's located right next to an airstrip called 40-Mile Air, so you might hear an occasional aircraft taking off early morning.
