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.
Never rotate my tires and always get 45-50k out of my rear tires- and that is all-terrain or mud-terrain tires which wear faster than the stock stuff. Front last about 30k but again- I don't rotate.
I run 50 psi in the rear and for long trips I will fill them to 60 psi. My trailer weighs 23k loaded and has about 5k in pin weight. No way anyone ever needs 80psi in the rear duals.