I noticed on the Free Campsites Website that a "Flying J" was shown near Kamloops on the Trans-Canada highway. We have often stayed at truck stops, including Flying J, Pilot, Petro-Canada, Love's, and others. Some of these large truck stops have special RV parking spaces so that you are not parked among the semi-trailer trucks, some have sanidumps, potable water, and propane, and all have gasoline, diesel, restaurants, a few groceries, and showers. Overnight parking is free, meals are generally inexpensive, and fuel less expensive than other brand-name gas stations, but showers may be relatively expensive.
Generally you will not be able to put out a slide or awning due to the narrow width of the spaces and we don't put down our steps either. But, if all you need is a place to sleep and you have gotten a good pair of earplugs at a Walmart or Canadian Superstore, you should have no problems spending a cheap night along the way.