All 4 tires (assuming you have 2 axles) are worn on the outside edges? Can you post a photo?
I'd say that if it's worn on the outside edges, you've got a positive camber problem:
And your axles are bent too much, as in figure 4 of this link (shows positive camber):
http://patentimages.storage.googleap...218-D00002.png
I'd think this is unusual, there should be some positive camber built in to counter the natural bend of the axles as the trailer gets loaded. As the trailer loads, the camber is going to tend to go negative. If you look at the 7k tubes on my trailer, they are intentionally bent "up" creating camber on the wheels.
There are a few $30-$50 tools to check camber, but if it's off, especially positive, the "fix" is probably going to mean replacing that axle with an axle that has the right amount of bend.
I would expect an alignment shop to be able to check camber, but maybe they don't as it's usually non-adjustable on trailers. Another possible cause would be running the trailer completely empty / unloaded for long periods of time, which would mean that the axles retain most of their natural positive camber instead of going more neutral due to trailer loading....