Kaplan tests are much more difficult. Go to the ADA site. It outlines specifically will be on the test. Therefore, they aren't going to put something on the test that is not on their list. This will save you a ton of time.
http://www.ada.org/prof/ed/testing/dat/scope.asp
Some of the topics are very broad, but they will save much more time then reading the whole Campbell book.
You can also get a test from the DAT publishers that best represents what to expect, and it is nothing like Kaplan. Although, I would very much recommend studying Kaplan material, because going that in depth really helps.