I had this error with my 18' fishing boat trailer. Got my first recall on the truck a week later to reflash heated mirror and integrated trailer relay module (FSA 21B08). After I completed the service, it fixed the issue.
My local camper dealer (DFW Camper) says the only ARE model available for the '21 is the V. They quoted me $3,285 (installed) for one that I built on ARE's website with windoors on both sides and sliding & tilting glass between truck/topper (tilting so you can clean the exterior side)...