Last year in May I went to both KI and CP during one road trip, it was well worth it for two days at each park as many coasters are excellent at each. Despite many of KD's coasters being similar to KI's, both parks as different enough to warrant the trip to KI at least once. KI's unique rides such as the Racer, The Beast, Diamondback, Flight Deck and Firehawk are enjoyable and worth many re-rides. Try to use early ERT there to get in position to make your way to be on the first couple trains for Firehawk when KI opens to everyone else. Firehawk has only two trains, is popular, and the wait time gets, then stays high unlike any other ride there besides Invertigo which is just brutal and not worth it.
Also, the drive between KI and CP, if you go slightly out of the way using the interstates, is almost 5 hours, so you'll want to keep that in mind and try to do it in the afternoon when both parks are the busiest so you end up in the other park during the night hours. The Platinum Pass gets you all the perks at each park like ERT, so it's best to take advantage of it's early morning hour as those rides are very busy during the peak of the day and will let you cross them off your list.
The kings island resort and conference center has a shuttle to the park every 15-30 minutes, it's restaurant has decent food and that hotel is very value driven, there are a lot of restaurants and fast food places around that area of Mason within a couple minutes by car. If you're staying at CP, I recommend Hotel Breakers for location, just keep away from Breakers Express. The Express' south side is near the train tracks on the mainland which you may hear all night and defeats the purpose of staying at one of the park's resorts as you have to cross the bridge to CP with everyone else.
I haven't been to Knoebels or Kennywood, but both are excellent parks from what I've read and saw online every time I looked. I agree with Matt that Knoebels should be included if you can do it as it's right off I80. Kennywood is more out of the way in comparison. Regarding Knoebels and the long drive to CP from NJ, I think you would want to spend a night near Knoebels and head to CP the following morning to "recharge." I always declined the stop there because the drive at night to CP would be more difficult to find compared to the "right off the interstate" convenience of KD and KI.