Actually, the Going-to-the-Sun Highway in Glacier NP is very rarely open before mid-June and never in May. We know because we tried it over Memorial Day weekend a few years ago and got the facts from the Park Service. And it closes for the season the day after the middle weekend in September.
Get reservations at least a couple of weeks early for Banff and Jasper if you want electricity. Most Alaska-bound travelers do those two parks on the way north, so they are much fuller in June than in late August.
A good FREE campground just across the Alaska border from Beaver Creek, YT is the Tetlin National Wildlife Refuge. TAKE WATER as it is not available at the Visitor's Center or campgrounds.
We got reservations for Denali 4 days before we got there because we didn't have a firm date until we got back to Anchorage from the Kenai.
Camping along the highway in Alaska is legal unless posted "no camping." Many Walmarts (including Whitehorse) and the Fred Meyer grocery store in Soldotna allow overnight camping. Several Fred Meyer stores also have RV dumps and many of the gas stations in Alaska, especially Tok, have dumps that are free with fill-up.