It's all about elevation in the mountain states.
Keep in mind that Boise is very hot in summer. Ambassador RV in nearby Caldwell is very nice.
Twin Falls to Pocatello is another hot area.
Hwy 75 north from Twin Falls to Stanley is a lot cooler and a nice drive. The Sawtooths have many great forest service campgrounds. Even our 40' motorhome fit in them. Stanley to Salmon and north to Missoula is a beautiful drive with cool temps and also nice forest service campground all along the river with excellent fishing.
Cascade and McCall is a cool temp area but continuing north to Lewiston you'll be getting into hot weather. Then it cools again at Coeur d'Alene/Sandpoint.
Idaho is one of our favorite states to tour. Have fun!