Get a copy of Mountain Directory West. By my reading there is only one pass in Montana that has a 6% grade downhill going west. If you use the trucks engine braking 6% is a piece of cake.
We drove Route 2 some years back. Don't let your gas gauge drop below half and don't expect large campgrounds with full facilities; swimming pools, miniature golf, etc.
I'd do it again. We pull 7500 lbs.
Dick_B
2003 SunnyBrook 27FKS
2011 3/4 T Chevrolet Suburban
Equal-i-zer Hitch
One wife, two electric bikes (both Currie Tech Path+ models)