Which of course is McCulloch's handwriting, and while it may be unique, does not confirm that he got Maj. C???'s name right. As an officer with significant quartermaster background, Cabell would seem to be a candidate here. Moreover, if there is no other reference you've been able to find in your reseaerch for a Maj. Campbell, a significant rank all other things considered, this may very well be just a spelling issue.
As you know from your work in census records, spelling is not a strength for most of our ancestors. Do with this what you will, but if it were my research project, I might conclude that McCulloch (or his secretary/adjutant) just got it wrong.