@ia_camper is correct. The company policy seems to be no overnighting. And I’ve seen signs posted at a few of the Buc-ees I’ve stopped at specifically stating no overnights. That said, you could always go in and ask the manager and they might accommodate you.
If you’re looking for locations to overnight for free check out this article I wrote: https://camperfaqs.com/what-stores-allow-overnight-rv-parking. I contacted 15 stores to see if they would allow Rvers to overnight at their locations. I should probably add Buc-ees to that list since they are growing in popularity.