If you visit the RV/Motorhome Hall of Fame and Museum in Elkhart first, they can give you a sheet of all the RV factories that do tours, and where they are located. Almost all are within about 25 miles of Elkhart, usually in small, rural towns.
Be aware that many/most tours are by reservation only. You have to call them and schedule a time. The Jayco tours run each day (not Sat or Sun) at about noon, but you still need to call and let them know you are coming.
The factories are assembly line and they do a certain model for however they need to make and then switch to another model. If you have or are thinking about buying a specific model, you can call them and sometimes they can tell you when they will be making that model. So for example, if you are flexible, you can schedule your trip in September, when they may be building the type you are interested in.
We just did the RV hall of fame and Jayco tour a few weeks ago. Highly recommend both.