The pre smogged diesels will be loosing their value at a greatly increased rate as they become older and worn out. Their features and creature comforts are now getting very far behind the newest offerings. To move any load be it shopping cart or camper the heavier the load the more energy it takes to move it. Diesel vehicles are not exempt from that law of physics. Modern diesels are suffering greatly in their fuel efficiency compared to the older engines. What is this diesel vehicle you have that gets such great mileage at 80mph?