I have the same problem with my Palomino. It has the rubber torsion suspension for each axel. I run through tires in about 5,000 miles. They only wear on the inside. I have taken them to the tire dealer for remounting on the opposite rim which gives twice the life.
The live axels are mounted on the larger axel that contains the rubber torsion bars. It could be that it is not straight. However, I would expect a twisted axel to wear one inside and one outside or to pull funny.
Also I wondered if overloading the PUP might cause the trailer axles to bow slightly making it wear more in the inside. Unfortunately there are just guesses. Also, I just purchase a TT so I am not using my PUP much, but it is too handy to sell.
Sorry I do not know the cause but when you find out please pass it on.