We live near Boise and have gone to Glacier/Hungry Horse several ways. The direct route up hwy 95 all the way to Missoula and Hungry Horse.
We've also driven Hwy 55, over the Banks Lowman hwy to Lowman and hwy 21 to Stanley, then 75 to 93 to Salmon and up Missoula, then to Hungry Horse.
Boise to Stanley is a good route for the person here who said not to do it. It is a mountain route, but with the right RV, tow vehicle, and experience, it's a beautiful route, but not as direct as through hwy 95.
Enjoy your trip.
2011 Bighorn 3055RL, 2011 F350 DRW 6.7L 4x4 Diesel Lariat and Hensley TrailerSaver BD3, 1992 Jeep ZJ and 1978 Coleman Concord Pop-Up for remote camping
Dave & Renee plus (Champ, Molly, Paris, Missy, and Maggie in spirit), Mica, Mabel, and Melton