The biggest reason for one tire to wear on the outside is the driver "pinches" the bottom of it against a curb in a turn. Usually that typically bends the spindle on the FRONT tire...when moving forward. And that is the tire you describe.
A good large trailer alignment shop can straighten it. Plan on $400 for the alignment and fixing the issue.
You might want to look closely at the other tires, to check them as well. If the tire on the other end of that same axle is wearing on the inside, that indicates other possibilities. Like out of perpendicularity.