Suggestion: once level and at cruising alt, trim to level flight, let go of stick and keep wings level with rudder pedals only. Bingo! She settles right down and flies straight. What the PROBLEM is that you are experiencing is that you are cross controlling and don't realize it. Thats why the heavy rudder requirements to one side or the other. You either have too much aileron or rudder in one way or the other.
Try it with rudder only. The ball will center nicely and you won't even have to touch the ailerons!