Gary...you are right, in that with the long weekend, it is going to be busy for traffic. Are you set on using a campground? Because if not..just go until you are done for the day and then look around. I drive 16 often and finding a place to sleep is never a problem. Having said that...I don't ever use campgrounds anyway. Every single city, town, district on 16 has commercial spots and I can't imagine you'll have a problem. Most folks on Labour Day weekend are home or in PG shopping for school stuff.
Burns Lake has a community CG, as does Smithers and Kitwanga...just for starters.
Gary Haupt