X3 on Four Mile Creek. I don't think every site had water hookups when we were there in 2014, but there were water sources in various places around the CG where you could pull up and fill up. (Maybe things changed or it could depend on the section.) Right on Lake Ontario. Very busy SP campground, family oriented.
If you have a dog with you, you'll be asked to produce a rabies certificate. (Obviously, it's important to keep a copy of all the records of shots, etc in the RV all the time anyway, but easy to forget before a trip.)
Fort Niagara State Park is near there, with the old fort open for tours, a visitor center, costumed interpreters, etc. - well worth a visit.
And of course the main event, the falls themselves, are a short drive.