Try keeping the 5th wheel well greased. Believe it or not lack of grease can cause about that same wear pattern on steer tires
Alignment out of speck when all else is good usually gives more of an even accelerated wear. This point to some sort of mechanical issue like mentioned by the other posters. I must add loose wheel bearing as a possibility.
Tandem tires makes a difference as well. If you have tires choppy on one side or mixed in with new tires. < Affects the alignment going down the road. You should feel it in the seat.
When the same wear pattern is on inside or outside of both tires and you can feel sharpness on the edges running your hand across the tread face then it's a toe in problem.
Ok, we went through this to the point of "getting" a frame straightening. Stop, before you spend any more money. Check the cab shocks and mounts. One of ours was frozen and pushing the cab forward. As soon as we fixed that, it went away, and we have not chewed up any more steer tires.
