Hmm Larry, If both tires are wearing on the inside only, it sounds like (in vehicle terms) what's called camber. There really shouldn't be a toe in/toe out problem with a trailer axle.
The axle itself, could possibly be slightly misaligned (the left and right hand sides not bolted equal on the frame), but this should result in one tire wearing on the inside, while the other wore on the outside.
Just guessing on possible causes of camber misalignment (which is where the top of the tire/wheel and bottom of the tire/wheel are not straight perpendicular) for a trailer axle, would be loose bearings....or a bent axle on each end.
We encounter this latter problem on our tire transport trailers, that we always are overloading.
The axle will bend/warp up from the point where it is bolted to the springs and outward. Once it does this, each end isn't straight with the axle anymore, and the camber is out. Ours will get so bad that the top of the tire will actually start rubbing the inside of the fender welds. It is at this point, that we replace the axle.
I wouldn't think your axle should be bent, but it might be something to look at, while you're waiting for Dexter to reply.
Edit: I found the following on the FAQ section of Dexter's website. It is just some good info that everyone might need to know in case they have a flat tire. Dexter doesn't recommend jacking up the trailer under their axle. Dexter also states in this FAQ, what possibly may be wrong with Larry's trailer
MISCELLANEOUS - Is it okay to jack up my trailer?
Dexter recommends that you do not jack up the trailer on the suspension components because there is always the potential for damage. Bent hangers, leaf springs, or axle tubes can cause bad axle alignment with bad tire wear resulting. Also, many trailer builders do not use Dexter hangers and we have no idea how strong these hangers may or may not be. Therefore, we take the conservative approach and recommend jacking up only on the trailer frame.
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http://www.dexteraxle.com/faq_s