So LouLou...As we understand your post, all four of your tires are wearing prematurely on the inside edge of each of them. And it is not an alignment, or overload condition. Just how many miles do you have on these tires? What is size and load rating on the tires? And the load rating of the axles? And the scale weight of the trailer?
Are you sure all four tires are wearing on the inside edge by an equal amount? Or are the front wearing more than rear or visa/versa? Or the left two wearing more than the two on the right? Visa/Versa?
When you went to the alignment shop, was your trailer loaded as it would be for a camping trip? All the clothes, groceries, drawers and fridge full, and storage areas with chairs, charcoal grill, etc. Or was your trailer empty as it would be before or after the camping season?
There is a very explainable reason for any uneven tire wear issue. Especially when all tires are wearing inside edges. And if you see that wear developing on tires that have only a few thousand miles on them. If your tires have 25,000 miles on them, wear on the edges is a whole lot easier to see. And may not really be all that bad a situation. But your post here leaves a lot of information open for speculation.
I gotta say that for all four tires to be wearing prematurely on the inside, it should be very obvious what the problem is. And if you want really want to know the answer, I think you would benefit from a second opinion.
I hate to see you spend a lot of money for a new set of good tires, and wear them out quickly. I suspect strongly, that the tires are not the problem.