You should expect to average 30mpg. I do, and I don't drive like an old granny either
Something's wrong. Check the simple stuff like Hatterasguy recommended. Have you checked for fuel leaks? Little leaks will decrease your mileage. I could have sworn my mileage went up once I replaced my leaking injector return lines.
To adress the sluggishness issue, clean out the lines and fittings to your ALDA. But first of all, drive the car with the hose off of the intake manifold, and check if it's even slower. If it is, then that's not your problem. If not, clean 'er out!
Here's a vacuum diagram PDF for MB diesels of the early 90's. Go to page 24.