I was curious what happened to the King's Island Inn (that motel that the Brady Bunch cast stayed in the 1970's) that used to be across the street from King's Island?

there is the Kings Island Resort, not sure if that is the same thing. It's located right next to the movie theater across the street and is still open. lol they had a "Reptile Show" this weekend

It is still there. I have stayed there a few times over the years and I can share with you that parts of it seems very run down and other parts of the resort kept up to date. In my opinion it was hit or miss on getting a decent room. For the same money I have found staying elsewhere in Cincinnati including just across the river to be a more comfortable and fun way to stay in the area. During the summer months the motel/hotel/resorts charge much more the closer you get to Kings Island so you need to just decide on the convince of being closer or for the same price but further away many more comforts and perks. We also have been known to stay on the north side of Cincinnati but further away from Kings Island to save money to be spent on splurges at the park.

Before the grand and glorious Mr. Lindner, Kings Island owned the inn and conference center, the golf course, and a lot more land than it does now.

What is it called now? Has it been changed much since the 1970s?

It is now called The Kings Island Resort and Conference Center. Has it changed much since the Seventies? Not enough.