Your 2006 is most likely built on a 2005 chassis. Looking at rockauto's site, they show the wheel studs are 9/16-18.
https://www.rockauto.com/en/catalog/...,lug+stud,7680
The only Dicor Jam Nut supplier I can find that shows the size is Grainger and theirs is 14MM but that would fit an F450. My wheels are Alcoas and use the internal spring type of lug nut cover; I have lost 2 on rough roads and I believe the threaded type (Dicor) would hang on a lot better.
Suggest you give Lichtsinn RV a call and see what they say. Not sure they would stock these, but if they do you can be assured they will fit ..... they know their stuff there. 1-800-343-6255 and ask for parts.
Good luck in the hunt and pls post back on your solution.
Good times and safe travels to ya.