Almost every ad says in small print, "Specifications subject to change without notice."
The ad for Vista says:"Winnebago Industries’ continuing program of product improvement makes specifications, equipment, model availability, and prices subject to change without notice."
Therefore, they could install a switch on one unit and not on the very next unit coming off the line without any liability. (change of specifications)
You were also told that your model did not come with said switch, so installing one and expecting Winnebago to reimburse you for it is not realistic at all.