Custer State park to south entrance to Yellow stone NP

Look at going a little south then through Thermopolis to Cody for a nite or 2,plenty of campgrounds. Great place to see with the "western Smithsonian of museums" and nite rodeo. Still a pretty long drive but not terrible. Leaving Cody go through YNP only about 50 miles from Cody then south to Colter Bay another 40-50 miles or so. Not a bad mtn. pass from Cody to YNP and you would not be back tracking but a few miles.