re: "...i think i should be getting around 400 miles per tank...."

Four hundred miles would be over 30 mpg (depending on how "empty" you want to let your tank drop to.

I'll admit that, on a trip (and because I have a preference for certain brands of gas), I start to get nervous when the needle drops close to the top of the red zone. So that means that I've got about 12-13 gallons to play with. With that in mind, re your '84 (which is like my '84 240DL with n.a. engine, with 206,000 miles), I have NEVER seen anywhere near 400 miles on a tank, and I've owned mine since '87, with 40,000 miles on it. Mine's an automatic, though .... is yours a manual transmission?

The best I've ever seen is about 27 mpg on one leisurely trip, but that was many years ago -- more often, it gave about 25 mpg or less routinely in those days. Nowadays, using it as my daily driver, I routinely get ~21 mpg with "enthusiastic" driving; but even when I'm behaving, I can't do much better.
I also have two '93 240's with much less mileage, and these both consistently get ~25 mpg with my wife and my daughter each using these cars -- which comes to about 325 (maybe 350 miles, on a leisurely vacation drive) before needing to fill up (unless you like to live dangerously close to "empty", ha, ha).

So I think you're expecting a bit much for these cars. Extraordinary gas mileage was never a 240's forte, I'm afraid.