Fifth wheel Travel on I-90 from Montana to seattle

Get the RVParky and ParkAdvisor apps if you have a smart phone. You won't find too many forest service campgrounds anywhere in the west that will accomodate a 40 foot 5th wheel (I have a 35 foot 5th wheel and travel a lot in the Pacific Northwest). The only one I know of on Hyw 12 is the Powell Campground just west of the summit of Lolo Pass. It has maybe a half a dozen sites that would take a 40 footer (I stayed there about 3 weeks ago). Private campgropunds are another matter.