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Jasper NP camping
Banff NP camping
General rule-of-thumb is that the larger, serviced campgrounds have reservations and during the summer reservations are highly recommended, especially on weekends (note that July 1st and first Monday in August are holidays here in Alberta and most other provinces). Midweek, I've had luck getting a campsite without a reservation.
The smaller, unserviced campgrounds don't have reservations and usually work on a first come, first served, self-registration method. Another advantage (for Escape owners) is they tend to limit RV length to 27' or less (see webpage for details). I tend to prefer these campsites and roll in on Thursday evening and manage to grab a campsite, (not on a long weekend though) and there are usually few campsites left. I wouldn't count on grabbing one of these on a weekend. Rather I'd grab a campsite Thursday evening, stay for the weekend, then relocate if desired midweek. Also, as they're unserviced, make sure your batteries and water supplies are topped up.
Best bet if you really want to be sure of getting a campsite though is still to reserve online. Do try to make it, it's a wonderful place!