DO not take a dog to national parks. They are only allowed in the campgrounds. In Yellowstone they are not allowed on any boardwalks, old Faithful, anywhere. It can be. A minimum of a $100.00 fine if you take them where they do not belong. Please out them in a kennel. It is unfair for the animal.
With a rig as large as yours your best bet is private rv parks at approx 50.00 a night. Must Forest service and national park service can not handle a rig that size. I have lived in Yellostone the last 6 years and that is your best bet. There is a place called Yellowstone holiday rv park and marina and they are 13 miles from the entrance to the park and run about 40.00 a night.