We took Hwy 2 across last summer and you absolutely can't go wrong. Couple of nice Forest Service campgrounds around Skykomish Wa on Hwy 2 and then on the east side, we have stayed at one called Tumwater, just west of Leavenworth, (a must stop BTW). Hwy 2 goes thru Coulee Dam and we were able to park right in town and they said it was fine, but there is also a City Park there. Beyond Spokane and north is Pend Oreille Park, heavily wooded, older campground which we checked out be didn't stay at. It would be okay for an overnight
I can't help you with Idaho and in Montana, after Glacier we found a great campground called Downstream COE near Fort Peak.
