Ultimately, I suspect it has to do with the outright popularity of the park. Places where tons of people go have more demand for larger sites, and, lo and behold, there are sites there for large rigs. However, if you look at the NPS site, you'll discover that the really popular places like Grand Canyon, Yellowstone, and the like, are most definitely in the minority of places operated by the National Park Service (including both National Parks and National Monuments). When it comes to these less well-known locations, your chances of fitting in are a good deal smaller, even for RVs a lot smaller than the one you are considering.
:)
Lynn