Just made that trip from Pennsylvania across to Utah and north up to Canada and back thru Montana, 6400 miles in 17 days. Watch out for altitude sickness in Colorado, it hit me hard. Utah was nice, take the 'Golden Circle"- something like 8 national parks all within a days drive of one another. North to Idaho, Craters of the Moon NP. From Spokane thru Montana you will need at least 2-3 days, it is close to 550 beautiful miles. Glacier had close to 14" snow a couple of days and several roads are closed, as is Beartooth Highway. Coming east thru Wyoming is Devils Tower NP and then Mt. Rushmore in South Dakota and then the Badlands NP 90 miles south. Both locales have nice KOA's to stay in. Just watch out for weather issues. Enjoy.
Never in doubt, often wrong