Fuel Tank Vacuum Issue
 

So I just successfully drove my 85 300D from San Diego to Boston. The car performed perfectly, with one exception. Long story short, there was a huge amount of negative pressure in the tank, as evidenced by difficulty getting the cap off due to suction. The fuel tank actually collapsed - sucked in like a coke can with the air sucked out. This left only 4-5 gallons of my tank accessible, making my range 100-120miles. I put a vented cap on it and was able to take an air hose and inflate the tank back to form. The problem originated in CA and was fixed in Iowa. I drove it the rest of the way with no problems. After checking it today it appears that it shrunk back a bit, as there is still negative pressure even with the vented cap. I plan to replace the tank, seeing as the gauge doesn't work and the structural integrity is now questionable. However, what is causing the problem? Is there a vent on the tank that could be plugged?
Thanks

Even if you replaced the cap with a vented cap if it was not a vented cap for diesel service it will not vent enough.

The only cap that will sork for this is a diesel service cap from Mercedes. Tey are not that costly.

I have read of this but never seen it. Why a cap would wear out and cease to vent is beyond me, but apparently they do.

Underneath the car there is a tank vent line. You find a steal line with a little rubber bell shape on the end. Make sure the bell shaped thing is clear. I bet it has mud in it.

I have seen the tank suction problem happen on 1 Oldsmobile and 1 Cadallic Diesel cars back arounb 1979. Aparrently that little vent holes (not visable or perhaps there is a vlave inside) plugged up with dust/dirt over time.
The Oldsmobile owner told us to drill a vent hole in his cap. I did so and he drove away.

Ditto. This sounds like your problem.