Dave, if you've run your rig across CAT scales you'll probably find the rear axle loaded a lot higher than the front. Add the wear from accelerating this load from a stop and the scrubbing of the rear tires when the 5th wheel tries to push the back of the truck off the road on curves and it's reasonable that the rear tires would wear faster than the front. I had similar rear tire wear on a Ram 3500 dually towing a 16,500 pound 5th wheel. Rear tires were almost shot at 22,000 miles.
2015 Rockwood Ultra Lite 2604 WS. Rear LR, large street side slide with small wardrobe slide in BR.
2014 Ram 1500 Eco Diesel Laramie Quad Cab.