OP says wrote:
Is this common or have I done something wrong? Sitting in one place too long?
Yes too both.
A google on ST or especially Marathon ST tires has pages of reading from all different types of trailering forums (not just RVs).
I've pulled trailer for hire and to RV since the late '50s. Tire makers have always recommended rotating tires on a loaded trailer 45 or 90 or 180 degrees in a certain time period that sit for long periods. I don't rotate my empty equipment trailers tires as they have little weight on them however they never sit but a few weeks at a time.
My 5er can sit from Nov to March/April. I rotate then 45 degrees twice during that time period.
I don't use ST tires on any of my trailers because of the same type experience(s) you had. I use all P or LT on all of my trailers.