Niagra Falls Park?

While you're in the general area, if at all possible spend a day or two or three at Letchworth State Park as well.

Busdriver wrote:
Four mile creek,

X3

another option is Branches of Niagara RV Resort.

Four mile creek,

Four Mile Creek State Park is hard to beat for cost, convenience (if you have a toad) and beauty. It's like on the shore of the lake, easy 15 minute drive almost traffic free to the Falls.

Make sure you have a passport so you can cross into Canada as that side is suppose to be prettier, but I was very impressed and pleased with the US side.

X2

This is a bout the best one. They have large pull throughs. They have a small visitors center that can arrange transportation and tours. We did use them We explored on own using toad.
Make sure you go down through wind cave and Hurricane deck. Boat trip on Maid of the Mist is a one time must.
Niagara Falls Campground
Make sure you take passports and visit the Canadian side. Parking is costly. We did by toad and also walked across.