I would try calling Lichtsinn Motors parts in Forest City, IA and see if they can source it. If Winnebago no longer makes that part, maybe Lichtsinn has one or can tell you where else you can try. There is also a place near Forest City that used to specialized in discontinued Winnebago parts. Can't remember their name.
Another option would be to take the passenger side piece to a sheet metal shop and see if they can use it as a pattern to make one. It is aluminum. Any good shop should be able to duplicate it. Then you would have to get it primed and painted.
'07 Country Coach Allure 470 Siskiyou Summit #31578, Cummins ISL 425; 2014 Ford F150 toad; Air Force One Toad Brake.
Glen Allen, VA; Smith Mountain Lake, VA.