We used US 89 to US 189/191 (the I-15 to Brigham City to Logan to Bear Lake to US 189 to Jackson). It was a good route in 2010.
Never been to the Craters of the Moon, can't help on that. From the Campground Map, it seems it might have 4 sites which might work for you:
CG Map. "Pull-through sites for large RV's and tow vehicles: 7, 11, 43, 45....sites are available on a first come first served basis; no reservations are accepted."
We stayed a few days at
Massacre Rocks State Park some years ago. It proved to be a very neat place, very scenic with campsites on a 'bluff' above the Snake River with white pelicans flying & landing & taking off from the river.
10 miles W of American Falls, ID
36 miles W of Pocatello, ID
83 miles W of Idaho Falls, ID
Very neat place.
Massacre Rocks State Park is located off of I-86 at exit #28, 10 miles west of the city of American Falls, Idaho.
At Massacre Rocks State Park: Remnants of the Oregon Trail are seen easily there.
Register Rock, located two miles from the park, is home to a huge boulder that holds the signatures of Oregon Trail emigrants who stopped for an evening of rest before continuing on their journeys. A scenic picnic area surrounds Register Rock, where emigrant name are inscribed, creating a desert oasis for the modern traveler. Remnants of the Oregon Trail are seen easily from either end of the park.
