I had no idea it takes 4 months to build one.
New Horizons builds their own chassis, cabinets, walls, roof, sides, etc., which adds more on site time. Their production line makes about three 5th wheel trailers/month. Virtually everything can be customized, which also adds a lot of time to the build. For instance, if you want a slide that isn't in the base design, then they will reengineer the chassis, walls, wiring harnesses, etc to suit your requirement. It all takes time.
Our trailer is a Summit, which main components like the chassis, walls, slides, etc are not customizable, but you end up getting built behind a Majestic(s) that are customized, so you're timeline is at the mercy of what's in front of you on the assembly line.
Our 30' International was on the assembly line for about a week, then in the custom shop where the 16" wheels were installed for a couple days, and sat waiting to be towed to our dealer for about a week. We had the Airstream about two and a half weeks after it was started.