Let's approach this question from the perspective of hermeneutics. Does the truth of the Gospel exist in what actually happened during the life, death, and resurrection of Jesus? If so, we are immersed in an historical-critical project from which we may never emerge, but in that view it would be difficult to argue that the historical Jesus showed anything like a preferential option for the poor. 
Thus the second question: does the truth of the Gospel exist in the stories told by the four canonical evangelists, which have been considered revelation almost since the very beginning? If so, those who get to tell the story also get to shape it. History shows they have often shaped it to fit their contours. 
Finally, does the truth of the Gospel exist in a dialectic between Gospel and those who hear it, so that one informs and reshapes the other? If so, then Liberation Theology, though not perfect in its methods, has a better chance of leading us to truth than the historical church.