Place to pull off and sleep in the Spokane, WA area

If you're just looking for a safe, yet easy access from the hwy, spot I'd suggest Lincoln's Silver Dollar. I have stopped there numerous times for a break, food and/or fuel. I've walked through the free camp area and it seems exactly like what you're looking for. If you arrive early enough, I'd also recommend the forest service campground a couple miles north of the I-90 on Camel's Hump Rd a handful of miles west of St Regis. Nice spot with water available and pit toilets.