Kountryguy, I'd head straight on up I-15 to US 20 at Idaho Falls,ID. Take US 20 to Island Park, ID. and stop at Valley View RV Park. Good monthly rates for a high tourist area, nothing fancy, huge sites, friendly owner and work campers. You will be 13 miles from the west gate of Yellowstone National Park. When ready to head to SD. just continue on into Bozeman, MT. and jump on I-90 and peddle to SD.