I spoke with an experienced trailer repair guy (not an RV repair place, but a "real" trailer mechanic) about poor tracking in my son's 5th wheel. He said most of the time it's not the axle, but the spindle that is bent. You may want to check that first. The "axle" is just a hollow tube supporting the plate that the spindle is bolted to.
Also, if you do replace the brakes, check out etrailer.com - they have a complete auto-adjusting replacement brake with backing plate, magnet, and all the brake parts including shoes for $50-$60 each. Five bolts and two wire connections and you're done.