I went through that same section last August and it was a bear. The one south of town was in a gravel lot beside a yet-to-be finished convenience store of some sort. It did have some very nice porta-potties, though. Hopefully, it's all up and running now.
I have no explanation for why there is not better coverage, especially for a city that is now at 700,000 in the metro area,. Maybe Elon wasn't a Merle Haggard fan.
A logical place would be near the Costco at the Rosedale Highway exit at Highway 99.