Wing it.... Only places I would consider making reservations would be places you " want" to stay at. We usually make a reservation for a campground so we can have a shower, wash clothes, dump tanks etc... and then usually only when it is close to a weekend. And then we only make the reservations a day or two before we get to where we want to be.
We like a particular campground in Whitehorse and in Dawson City so those two we do make a quick phone call and book.
There are TONS places to stop over night in Southern BC just have to know where you can't park.
Places you can't park over night for free are.... Provincial and National Parks, private property, Indians land and most cities towns and villages. If been doing it for most of my 60 years so it is possible.
Also there are books you can buy called " Back Road Map Book" that have all the backroad of of some of the Canadian Provinces mapped out. On those maps ( talking BC ) they show BC Forrest service campgrounds which are usually down a gravel logging road at a variety of distances from the main roads. These forest service campgrounds are basic, and usually have a pit toilet and maybe a couple picnic tables. They a rough campground nothing fancy, but they are usually located by some beautiful little lake or stream. I have two books for southern BC as well at the chip for my had held GPS for all of BC. Comes in handy when we are out in the bush on our side x side.
NOTE: some of these roads are rough, steep, windy and narrow, so do some research before you head off 45km down a logging road.
BRMB also carry water proof maps so you can have either a book, or a map. The books are available on Amazon.com, however I haven't found the maps on Amazon, but I do know they ship to the States.
Back Road Map Book.Soup.