Wukkers, It's usually heat and sun damage that ruin trailer tyres.
Possibly a bit of overloading may become an influence.
Just removed 2 Coopers off my 105r Cruiser after a decent run of 180,000Klm.
They were the last of a set from 2012, and had 3mm of tread left, mostly used on bitumen, so I think that was the key.
Rotating them front to back every 10,000 seems to be the trick, provided you have them at the right pressure to wear evenly.