Playing by ear is a result of having a knowledge of theory. I'd suggest getting a "chord" book, illustrating the finger positions on the keyboards. It is helpful if you can learn how to 'invert' your chords...for instance a simple 'c' chord can be played 3 different ways. From the chord book, you can learn to 'flat the 3rd' for a minor, 'add a 7th note' in the scale for a Maj7, etc. I personally can say from experience that I studied like that as a kid for years... and with arrangers, it has become a great tool. If you read and then can add this approach to the arsenal, you'll be set. My chord knowledge is deep but my reading is limited and lame, which is also very frustrating.
Good luck,
Zuki