Many thanks to all for your feedback and suggestions! I'll incorporate each response into planning this trip. Since it looks like we should be able to travel the Magruder Road with our truck and camper, I'll be planning a trip for July, assuming the road is open (no snow, wash outs, construction, etc).
Our plan is to drive from Grangville, ID over the Magruder Corridor and down into Darby, MT where we will pick up the highway. We'll go from Darby up over Lost Trail Pass and down through Wisdom, MT, then over to Jackson, MT where we will get back on Forest Service roads to travel up over Lemhi Pass, going back down into Idaho where we will pick up the highway and return to Utah by way of the Sawtooth Recreation Area, Sun Valley, Twin Falls, and home to Springville.
Thanks again to everyone...Terry