I have 27,000 miles on Ram dually and the rear tires are worn down to less than half tread left fronts are fine is this normal with dually's. I always keep tire inflated to the reccomended pressure.
OEM tires are not the best quality and seem to wear faster than replacements. Also hauling a heavy load increased the rear tire wear. My rears wore faster than the front but I tow a heavy 5er.