Ft. Steele RV Resort was full when we went through the area, Sept. of 2012, but Ft. Steele Campground had spots available. If you prefer the larger private park setup for your month, I'm sure the Resort will suffice
https://www.fortsteele.com/rv-sites . However the Campground
http://www.fortsteelecampground.com/page1/page1.html is a family-run, older park that might do if you like quieter and smaller. It's back away from the highway and in the trees, but has limited space for large rigs; call ahead. Family members greet and help get you set up, and regularly make sure all is well. It's more primitive than the Resort, but off the beaten track and a bit less money. I'm not sure they could supply 50 amp, but this is not an area where air conditioning is a necessity.
Another option is Dutch Creek, further north toward Fairmont Hot Springs. The Hot Springs Resort was not my favorite, too big and popular, full to the brim with people, and we got stuck next to the highway, the WiFi was terrible and overloaded, and the staff overworked. Though it included the infamous hot springs, we were not there exclusively for that like most seemed to be, and it was not my cup of tea. We moved the next day down the road and discovered Dutch Creek, which was essentially a "permanent" renter/owner RV setup. They were in a period of flux at the time, as to overnighters, so I'm not sure of their current status, but you could check it out. They have large sites with plenty of space between neighbors, lots of trees, and you can easily hike and enjoy the flora and fauna and the raucous "creek" that seems more a river that empties into Columbia Lake, the Columbia River headwaters. An osprey nest overlooking the creek was cool to watch daily from our site. It was at the time a developing "resort", and we seriously considered buying a lot there as a summer RV retreat.
http://www.dutchcreekresort.com/Regardless of where you park, the nearby town of Cranbrook is sizeable, with every amenity and service you'd need, and Ft. Steele itself is one of the best and most accurate reconstructions I've seen. No wonder films have been made there. We enjoyed the horse-drawn grain harvest where I got an idea of how my Quebec farm-raised maternal grandfather must've farmed his Alberta prairie homestead. Closeups with Percherons and other workhorses made me feel closer to his locally famous steeds from the early 1900's. The blacksmith's shop at Ft. Steele, given I was a lone visitor that day, put me in touch with my Ashley-side grandparents' English/Nebraska occupational heritage. Then the 1800's bakery down the street sold Lee the only Whole Wheat Sourdough loaf we'd ever heard of; subsequent daily meals confirmed it to be our favorite bread. None since has been as good, even when we have managed to come across the somewhat rare whole wheat sourdough elsewhere. Fresh and warm it was to die for.
Methinks one could hardly do better than staying a month around Ft. Steele. There's also some RV parks in Cranbrook itself, although to my eye they weren't quite as intriguing and more just places to park in the midst of the large town's conveniences. It would be a last resort for us, but at least an option if everything else is booked.
Joel