Does the weight of the trailer count towards towing capacity?

Of course it does. If you think it is close, go to a truck stop with a scale A: weigh your whole rig (all 3 axles on the scale), B: just the rear axle of the vehicle and the trailer C: just the trailer axle. D: Remove the trailer and weigh your towing vehicle. Calculate: E: The loaded trailer ( A minus D) should not exceed the towing capacity rating for the vehicle or the weight rating for the trailer. The load on your rear axle with the trailer (B minus E) should not exceed the weight rating for that axle THIS IS VERY IMPORTANT: have you ever seen trailers swaying on the highway? this is very dangerous. Your trailer tung weight - the weight on the hitch (E minus C) , should be about 15% of the weight of the trailer. That bumber cannot exceed the weight rating of the hitch, but if it is too small the trailer can be very difficult to control.