We were just through there (not with the Montana) about a month ago. It would be very tight even driving through the campground. There were only a couple of sites that you could fit in. The sites are first come, first served so you can't reserve a specific site. The park ranger told us that it's really not a big rig friendly park. Even if you don't stay there it's definately worth several hours seeing everything. There are several reviews at RV Park Reviews -
http://www.campgroundreviews.com/regions....html#CGID8687 . There were a couple of campgrounds between Arco and Craters of the Moon but none of them looked like we would want to stay there for a long time. There was a nice campground about an hour north of Craters of the Moon at the Mackay Reservoir on US 93. We drove through that campground and decided that when we retire we wanted to go there for a week or so. The weather looks to be in the high 80s and low 90s. While you are in the area be sure to go to the Experimental Breeder Reactor 1 (EBR1) just east of Arco on Hwy. 20/26 ~ 20 miles -
http://en.wikipedia.org/wiki/Experim...eder_Reactor_I . We took the tour and it was very interesting. It was the first electricity generating nuclear reactor. Good luck and have fun on your trip!