I've looked at it, but not stayed there. It's right on the snaky entrance road on the way into Mammoth from Gardiner. There are no hookups at all but you can run gensets in the hours you mentioned. There were larger rigs in there so it could be a viable parking area.
We stayed at Yellowstone RV Park
last fall and would go there again this fall if the trip materializes for us. It's 5 miles up the road and is located right alonmgside the Yellowstone River in Gardiner. It had full hookups available but you'd have to call them to see if they are open in winter.
I've stayed at Fishing Bridge but that's pretty tight spot. It wasn't the worst in our 33' gasser although having slides on opposing sides made it tight. It was nearly impossible with a 40' coach though and the trees were really squeezing us. I wouldn't do it again. If your RV is smaller it might work for you but it won't be open in winter because the only road open in the park to vehicular traffic is Mammoth to Cooke City. Then again, you could probably cut a deal to boondock in a motel parking lot in Cooke City also. Being close to the wolf packs in Lamar Valley when they're active in winter would be very attractive to me.