Fuel Level Sender
My wife's 1995 C 280 as well as my 2000 ML 320 has done the same thing on occasion. Thankfully, the fuel gauges have performed flawlessly for the last 1/2 year. Like yours, after a fill-up, the fuel gauge only goes to 3/4 full or will stop reading altogether. Usually after several fill-ups, the gauge goes back to working normal. I believe it is something in the fuel (dirt?) that gets on the fuel sending unit and until it gets cleaned off it causes an erratic reading on the fuel gauge.
My wife no longer gets concerned when this happens since she says it will "fix itself." You may want to try a different gas station or gas brand. For some reason every time we fill up using a certain brand of fuel (name unmentioned), no matter what service station for this brand we use, it causes the fuel gauge to act erratic. We no longer use this brand and since -- no problems.