The Grizzly is very nice and priced accordingly. West Yellowstone is a good base if you need full hook-ups. The KOA is pretty much what you would expect...
In the park itself, there is the Fishing Bridge RV Park and it has full hook-ups but it is kind of a parking lot with trees. Everything else in the Park is dry camping.
It is a great time to be making reservations. Full hook up sites are pretty scarce come late June on until late August.
See you there,
Old (usually dirty) Nissan Titan
Old (enhanced with "male pattern baldness") Sovereign
Young, lovely bride
Goofy dog, dismissive cat