Best Route Advice

We are looking at heading from Portland Oregon to Glacier National Park next summer, along with a few additional location (Athol and location near GNP). Looking for some route advice. Not necessarily looking for scenic, but rather best road with least amount of long inclines. Pulling at 38" Fiver with a modified F25D which tows in the mountains pretty well. I have been told by a few folks (not sure if they actually made the pull or not) that 84E to and then up to 90E is the best way. Not sure how mountainous that route is so looking for some advice.