If you're OK with driving mountain roads the route is just fine. We've done it in both directions. Also, get the 'Mountain Directory for Truckers and RVers' to read about the important areas.
From Spokane, the first one you'll come to is Lewiston Hill on US 95 just north of Lewiston, ID. Southbound is a steady 7% descent for 6 miles with 3 runaway truck ramps.
Next is White Bird Hill on US 95 south of Grangeville, ID. Southbound descent is 8 mi. of 7% grade with 3 runaway truck ramps. Alternates from 2 to 3 lanes. It has a 2-lane bridge at the bottom of the hill.
Then you'll get on 55 with no issues southbound.
Then you'll get on 21 east between Lowman and Stanley are some short 3-4% descents.
Stanley is where the awesome Sawtooth Nat'l Rec Area is.... don't miss exploring it.
From Stanley down to Sun Valley you'll have Galena Summit on Hwy 75. You'll have 6% grades for 8 miles.
All of these are very doable with a RV and you'll see many doing it. It's a popular area. Lots of campgrounds!! Enjoy!
Full-timed for 16 Years . . .
Traveled 8 yr in a 2004 Newmar Dutch Star 40' Diesel
& 8 yr in a 33' Travel Supreme 5th wheel