I've had this happen with two different motorhomes, 2 different chassis same tire manufacturer...Sumitomo.
On present motorhome, a freightliner chassis with 275x70x22.5, both front tires have wear on outside edges. This is after 4 yrs, approx. 40,000 miles. Just yesterday I rotated the front tires to the back. I plan on running these tires another 2-4 yrs.
Not sure what causes this, but have not seen it with Michlen or Toyo's that I have run. On a previous motorhome, I even had "rivering" in the tread on one tire, and "cupping" on another. Otherwise I've had good luck with Sumitomoes.
Just doing the one time rotation, I've been able to age the tires out.