It's called a wheel cover nut or wheel cover jam nut. The actual lugnuts are under the wheel cover. I found that the direct replacement is a Dicor V19582-EJN Jam Nut, but they appear to be unavailable. They fit both W22s and W24s. Here's a link to a substitute on etrailer:
https://www.etrailer.com/question-618228.html
To be safe, I'd verify the measurements before ordering.
If these aren't available, you might want to try posting on IRV2.com (Winnieowners' sister site) on their Workhorse forum. I believe you can use the same sign on as Winnieowners but I'm not sure if you just sign in or have to set up an account:
https://www.irv2.com/forums/f22/