If you like twisty roads, take HWY 21 from just south of Boise to Stanley. Stanley is right where you have the best views of the Sawtooths. Just 5 miles or so south of Stanley is Redfish Lake, right off the highway a few miles. HWY 21 is about the twistiest road I've ever been on.
The faster, easier route is over on the 20 going east then north through San Valley (Bellevue, Hailey, Ketchum). North of Sun Valley on the 75 the ride is great, going over Galena pass, then into Stanley.
From Stanley, going north on the 93 all the way up is excellent. Follow the salmon river, great riding until about Hamilton MT where then it's just a fairly populated corridor to Mizz, MT.
I wouldn't worry too much about motorhomes, etc. It's nothing like Yellowstone/Teton area as far as traffic.