If you're looking for a meeting place or a good stop around ½ or halfway between Casper and Sheboygan, you can try Sioux Falls, SD, which is almost 9 hours east of Casper, or about 7½ hours west of Sheboygan.
About ¼ or a fourth of the way, you can stop at Rapid City, SD. This is almost 4 hours northeast of Casper.
About ⅓ or a third of the way, you can stop at Quinn, SD. This is about 4½ hours east of Casper.
About ⅔ or two thirds of the way, you can stop at Fairmont, MN. This is about 10½ hours east of Casper, or about 5½ hours west of Sheboygan.
About ¾ or three fourths of the way, you can stop at Rochester, MN. This is about 12 hours east of Casper, or about 4 hours west of Sheboygan.
If you drive straight through without stopping at all, the total distance is 1,103 miles. The distances shown above don't include local roads to get into each town, so if you're planning a long road trip, make sure you factor in traffic and road conditions, rest stop breaks, and interesting detours along the way. If you're in a rush, you can do the trip with zero stops, and your total travel time would be around 16 hours, 2 minutes.