Idaho is a secret to most RVers. It's an amazing state to explore.
You mention I-84 and Hwy 20...don't stop there. Take 95 up to McCall and stop at Ponderosa State Park to explore.
Another area would be the Stanley area via 75 from Twin Falls. This is the beautiful Sawtooth Mountain Nat'l Recreation area. There are many campgrounds and many forest service campgrounds right along the Salmon River - excellent trout fishing up there and lovely mountain vistas.
The area south of I-84 and looping around on I-86/I-15 can be very hot in summer, including the Boise area. You need to travel further north in Idaho for cool temperatures.
Keep in mind that if you get to Idaho Falls it's not very far to West Yellowstone via Hwy 20 or to Jackson, WY/Grand Teton Nat'l Park via 26 from Idaho Falls.
...so many wonderful things to check out in the area. Have fun!