Grizzly RV is definitely well worth, but we prefer FS CGs and stay at Baker's Hole, just 2 miles north of WY. First come, first serve, so while at Grizzly we head over with our TV to Baker's Hole early and talk to the host. They let us know which sites are expected to vacate and we grab one, pay for it, put our name on the pole and head back to Grizzly for our FW. It's worked for us every time.
BTW, we've also stayed at West Yellowstone KOA and Red Rock, but Red Rock sites are close, quiet out there, but close. Hubby didn't like it there.
There's a lot of RV parks in the West Yellowstone area. Check out Campendium for reviews and pictures.