I have a similar problem recently on my Cobra.
The extreme outer edge of contact area of the left tire is wearing and cupping.
The right tire is fine.
The trailer is from around 1997. Aluminum top.
I estimate I am doing about 1000 miles / year.
Behind my Blazer, this trailer runs rock solid, even significantly past 65mph.
No swaying.
I contacted Spindelberger, who replied immediately and suggested to check the bearing, wheel balance and alignment.
The bearings seem fine. No roughness in the bearing and no knacking sounds.
Also the break does not engage.
I have had no problems backing up.
I still need check the balance, the tire alignment, shocks and boots.

Any ideas how/where to check the alignment ?
And how do I test the shocks ? The suspension seems very stiff.

Paul, I would not endorse your "Letter to Spindelberger"

3U.