Sorry to disagree with some of the other posters, but our experience was definitely not the same!!!!
Spent 3 weeks in BC and the Parks in Sept/Oct 2015. Lots of parks were closed after Labor Day, both the Nat Parks and the Provincial. We were able to find spots for every stop we made, but wasn't easy!! It was a beautiful fall, lots of colors, and lots of rental Class C's running around!! Many Europeans, many (can I say it??) orientals, a couple of caravans, and many private units!! It was very busy, a real zoo!!
Didn't make any reservations at any parks ahead of time, had to take what we found. Got a couple of nights at Redstreak NP, (only one loop open), found a couple of nights at Johnston Canyon near Banff (same, only one loop avail!!), only could get one night at Whistlers near Jasper (but fully open!!), and got a couple of nights at Wapiti open lot with elec (again, partially open!!). Rest of trip was in Provincial parks (that were open!!).
Hopefully you won't find it as busy as we did, but I'd be prepared!!! We were somewhat limited by our length (34ft trailer) and first trip up there, so we could only play it by ear!!
