I contacted Parks Canada in January this year as I will be riding the Parkway in June,this was part of their reply
"Whistler’s campground does have a loop of sites that are specifically for visitors who are in the park with no vehicle. These sites are only available on a first-come first-serve basis, no reservations would be available."
"Additionally, Jasper National Park has several self-registration, first come first served campsites along the Icefields Parkway and one on Highway 16. These campgrounds can only be reserved on arrival, in-person, at their self-registration kiosk. Checkout time for guests leaving the campgrounds and earliest arrival is 11am."
Roger