The weight limit on your RTT is going to primarily be based on the vehicle's requirements. Find out what your vehicle can handle. Beyond that a good rack system is obviously important.
Also, in the owners manual the weight rating is going to be what you can carry up there while you are driving. IE My Jeep Grand Cherokee can handle up to 150lbs. That means when I am going down the interstate I can carry 150lbs. When I am stopped I feel confident that it can hold another four hundred lbs just fine. (I figure between myself my wife and our son we are in that weight range)
Hope that answers your question.