Not a ranch, but Love Valley is neat little town in the brushy mountains of nc. It's set up like an old western town. It has a campground and there are other nearby campgrounds listed on I to website. It's mainly for those to bring their own horses, but I see there are horse rentals listed as well.
Two that I know of: Terry Bison Ranch in Cheyenne, WY. and the Mt. Rushmore KOA, SE of Hill City, SD on Rt 244. Both are RV parks, so I doubt that they are "working ranches", but they do have stables and offer horseback riding. Running RV Parks or Ranches are both more than full time jobs.
Retired and visiting as much of this beautiful country as I can.