Although I haven't ridden this route, I grew up in the West Kootenays and know the area fairly well. There will be very few opportunities to find a motel but many to pitch a tent; although be prepared to do some stealth camping. There are sporadic Forest Service Recreation sites, (informal camp sites) along the way but most require a trip off the main highway to get to. The route from Vernon to Nakusp is very desolate (192 km) and the most you can hope for along there would be a gas station for food and water.
After Nakusp, there are several side loops you can make (Nakusp-Slocan-Nelson-Salmo-Castlegar) or (Nakusp-New Denver-Kaslo-Nelson-Castlegar) or (Nakusp-Slocan-Castlegar) however, I would highly recommend the inclusion of Nelson on your journey.
The next leg of the trip from Castlegar-Christina Lake (you should spend a day at the warmest lake in BC!)-Grand Forks-Rock Creek will have many motel opportunties. At Rock Creek, you'll have to decide if you want to ride the 33 back into Kelowna or take a more circuitous route through Osoyoos-Penticton.
Highway 33 will be long and no opportunities for finding a motel before Kelowna. Around the area of Big White Ski Hill, the road narrows and there is a twisty descent for several miles. You will encounter many big rigs and logging trucks on this highway. However, the route through Osoyoos-Penticton also has it's share of heavy truck traffic as well as a large contingent of holiday 5th wheels and the like so it's really a coin toss on that one.
The number 3 highway is a main trucking corridor to the west coast, so you will have to contend with this; however, there are wide shoulders which off-set this somewhat.
I hope this is the kind of info you're looking for. If you have any questions about the area, PM me and I'd be happy to share what I know about the place.