I am not sure I understand your math. When you draw a free body diagram of the trailer and towing vehicle, you would have 4 axles/8 points of contact with the earth/ground where weight is transmitted to the earth/ground or DOT scales. Depending on where the trailer payload is located will vary the 8 points of contact. If you transfer weight to the towing vehicle to have tongue weight you are taking weight off the 7K trailer axles and tires. It all needs to be balanced though to be legal and safe.