I wouldn't bet on Keystone stepping up to pay for a new axle. You have used the RV on unknown roads and these roads in North America are getting worse by the day.
If I needed an axle, I would look at Eastern Marine first (
https://www.easternmarine.com/em_store/axles ) they sell Dexter axles and are quite a bit less then anywhere else that I've looked, You will have a part number tag stuck somewhere on that axle, or if washed off, the numbers will be stamped on the back side, Eastern also show you how to measure for a double check.
It's a very easy change out that a DIYer with a few tools can do or even a local and decent garage if you don't have the tools. The tools are a ratchet and 4" extension 13/16" socket for the wheels, a 3/4" socket for the U-bolts and a 9/16" socket for the backing plate bolts along with a big pair of pilers for the spindle nut. You can borrow a torque wrench from Auto Zone or Advance Auto Parts. If Harbor Freight has a nearby store, 20 bucks will buy all but the torque wrench.
Good luck with your dealer and Keystone but also let us know