I have always gone in from the south-east. From Casper it is a long but magnificent ride through the plains of Wyoming and then through the Wind River reservation. We always stay in Dubois at The Longhorn Ranch park right on the Wind river. From there we go west to Moran Junction which is between the two parks. Going in from the south of Yellowstone is no problem for us. We are 37ft plus a toad. Have a good trip and soak it all in. Yellowstone is our favorite place. We'll be back this summer!