It is a fact that axle hangers are often installed wrong..... But since you got 18K on the tires, that can be ruled out... Reason being: If they were misinstalled from the factory, then they would have been wearing bad from day one. Since you got 18k before noticing it, my money is on something that happened later.
Perhaps you overloaded it one or more times. Or hit something that bent them. Those are the most likely things. It is easy to do, as manufacturers put the lightest (cheapest) axles on that they can. At any rate, an axle swap is easy, and they should be able to handle it just fine.... Do get it weighed ready to camp, before you take it in... Just to be sure you are not overloading it. Be a shame to bend the new axles on your first trip.
Inspect the equalizers, as they are a wear item. If they show any signs of wear, this would be a good time to replace them.