Jason:
The V10 in our Ford E450 has handled both the East Coast & West Coast mountain ranges without difficulty. I generally drive the speed limit, or in a few cases, just below. Our worse mileage was 7.5 westbound on I80 in southern Wyoming during extreme high winds, our best was 11 with 40 mph driving on the main roads in Yellowstone. Generally, we average about 8.5 to 9 mpg cruising the Interstates.
One tip I might offer about driving/parking in smaller towns and cities. If, for example, you are going to a museum or eating out, call ahead. Tell them you want to visit their establishment, and ask them where is the best place to park your _______ foot RV. This has worked well and has saved us from spending a lot of time and anxiety searching for a legal place to park.
BTW, are there camp sites you would recommend in or near Missoula?
Bob