When booking on the websites such as ReserveAmerica (which now also looks after reservations for a lot of Canadian Parks) on the site selection drop down the 18' & UNDER is specificity to the Equipment/Trailer only. Most do allow for the additional space required by the vehicle to park if its not clearly stated as tents only.
On ReserveAmerica for example when you select a site from the list of available sites, on the pop up there is often a link to check the site details which will normally include the sites length & width & some will show the actual trailer/tent pad length as well as the max driveway length. Some include photos of the sites. Sites for 18' and shorter trailers are often in the 45' total length range -some may be a little more some a little less which allows just enough room for a 18' trailer and a standard length vehicle (16') and a few feet between the front end of the vehicle and the road way.
BTW Frederick your item "#3 Dispersed water taps, no electric / XX foot limit" pretty well covers most Provincial Parks
although they are starting to put in power and water into a small number of sites at some parks. I recall having a giggle a few years back when our then Provincial Premier was asked why our parks dont have water and electrical
at each camp site and his response was our tourism slogan is "Super Natural British Columbia" and if people need power and water to camp they should just camp in their driveways.