Bent Spindle. I hade the same issue seems these Cedar Creek 29IK left front tire only, inside thread only.. I did not notice the wear until after about 2k miles. Getting a look under the rig revealed a loose leaf spring clamp nut also "Yikes". Had the rig checked out buy a trailer service center to perform a Laser Alignment.
The bad news was a bent spindle, But I had taken photos (cell phone) as proof of the loose parts. I made my case calling the warranty Rep at Forest River that the assembly was not properly installed when the coach was built and since the causation could not directly be established with any certainty they agreed to replace both Axles. Forest River did have to mediate this process with Dexter manufacturing. I've come to the understanding that the Axle assemblies come into (Cedar Creek) assembly plant fully assembled and they essentially bolt them in place.
Remember that when you have issues take photos they can go a long way with regards to Warranty issues.
Don & Rebecca
living can be broken down into two elements
Define the essentials
Eliminate the rest