My bet is that bear visitation rates change year-to-year, and season-to-season. Hard to say now where the bears will be in September.
I frequent the Good Sam/RV.net forum, and there are a
few recent threads on Yellowstone. There are mentions that most Yellowstone campgrounds are without power.
Check the threads, and be sure to check the
Nat'l Parks reservation system for details of each campground. They will say whether there are sites with power, and if they are available for reservations.
Good luck, and enjoy your trip!