Having made the long drive over and over I know some good spots to go pee when you're on that drive.
This is my favorite because I can get out of the car and go look at hunting, shooting, and camping gear. I believe they have toilets inside as well.
Good food in the restaurant, gift shop full of trinkets and shirts and stuff, and a full grocery store that sells ammunition. The bathrooms are between the gift shop and the restaurant.
Side of the Road - Mile Marker 181
There is a farm in the distance to the north but it is about a mile off the Interstate. If they had binoculars they still wouldn't see much. I also like peeing at mile markers that have a symmetrical number like 181, 232, 303, etc.
Scenic Overlook Between Kadoka and Phillip
There is no toilet. No porta-potty. I don't recall that there is even a hole in the ground. But do you think you would be the first person to pee up there? No. And there is a magnificent view of the Buffalo Gap National Grassland.
Anywhere at Wall Exit Except Wall Drug
There are a couple of gas stations and restaurants right off the Interstate in Wall. They all have a place to potty. Wall Drug does too, and it's a fine place to visit, but if your objective is urination it's a long trip into town. Like 600 yards or so.