I've seen a few other reports like this and suspect that there can be conditions where towing does indeed affect the geometry of the toads front end. Slop in receiver or towbar could be a factor. So could some variance in the attachment of the base plates to the car. Lastly, the front frame horns where the basepaltes attach to the toad is not always real strong and stiff. They could distort a bit under towing loads and make the front tires act differently than they do when under power. And one brand of base plate may work a bit better than another for any given model of toad.
Since the reports of this problem do not seem to be consistent for a brand & model of toad, I would guess it is a combination of the above factors. Toad alignment may be a factor too. There is a fairly wide tolerance in alignment specs and a shop may report "in spec" when in fact the front end is somewhat skewed. It may be useful to get the alignment set "dead center", or maybe even an opposite extreme. If I were getting inner edge wear, I might ask that the toe-in be set as far "in" as the spec allows, rather than just anywhere in the allowable range.
All that said, scalloping indicates the tires is hopping a bit, so that it does not wear smoothly. Since both toads did the same thing, I'd look closely at the receiver and towbar.
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL