In BC Park Campgrounds, Campsites are available for anyone, doesn’t matter the size of type of rig, with the exception of Handicapped Sites.
A lot of the time people coming into a campground might only find the one site is available and it is a large site, so that’s what they take. Other times it might be the location of the site, like right on the lake shore so it’s taken for that reason.
The BC Parks reservation site map when clicking on a particular “site” will show the size of the site. And in the reservation system when you fill out your profile and there is a box where you add the in your rigs type and length and when you enter a site number, it will show you if you will fit.
You don’t have to necessarily book a reservation, but you can use the system to help you find the sites that will be big enough for your rig, then you would have a list of sites to drive by to see if they are available or by using reservation system see if it is already full before you even get there.
As well a “lot” of BC Park overflows are basically a gravel parking lot or a field which will also be available if required, and that if the campground has an overflow.
You can’t just boondock in a Canadian National Park or a Provincial Park, you must stay in a campground. Southern BC and south western Alberta are very busy in the summer time for camping, so just driving up to find a site, May already be putting you in the over flow, for the more popular campgrounds. Once you get about 1/3 of the way up BC and Alberta things open up more for vacancies in campgrounds and places to boondock.
Prince George BC and Edmonton Alberta are just over 1/3 of the way up the provinces.
I pulled a 35ft 5th wheel all over western USA and western Canada and I didn’t ever find a place that I couldn’t park, but I did do some research.
The biggest issue and one that ever person with an RV should remember is that a lot of theses campgrounds are in the Forrest, and with that comes low hanging branches. In my truck and camper I’m inches under 12 ft high, and I am for ever looking up and around when driving through campgrounds for those low hanging branches. I have been bitten by some of those branches before, so I am talking from experience, so heads up.