Diesel MPG

hi I have a 1982 Mercedes 300D Turbo Diesel 4 door sedan. The car has a 1985 turbo diesel engine in her, with 50,000 miles. I changed the oil 2000 miles ago and it still doesnt need any oil, it's unbelivable. Well my question is how many MPG am I supposed to get? From full to half tank i get about 150 miles. Once I reach like 250 miles im nearing empty, I dont know why. These cars are supposed to go 600 or more miles in a tank full. Is there any reason this could be happening. I know the alignment is real bad and I need new tires that I will be getting later in the summer. But How many MPG can I really lose from that? it pulls to the right. The trunk smells like diesel could most of the diesel be leaking there or something. I know I have to replace the rubber thing in the gas casket, because its puntured, thats why diesel is leaking into the trunk but thats only when the diesel oil reaches to the casket, and im guess that happens when the tank is full, how much could it leak from there? Since the car has a 1985 engine, does that mean the car has a trap oxidezer, what is that a special air filter or something, and how can I tell if I have it. My tranny is rebuild, and wondering how is it supposed to feel? Sometimes it is as smooth as it can be and other times, you can feel 2nd gear kick. Mostly when the engine is cold or im pushing the pedal lightly, depending on how I push the pedal.