The instrument panel gage sensor cannot cause mileage to drop by failing to register the correct engine temp, can it? I did not think this sensor "spoke" to the engine controls.
My 1988 300E runs just below to just above the 80 degree Celsius mark, and never goes higher. While driving the car home from Utah to Connecticut I got 29.8 mpg or so and figured the temp was normal for the car. Did your car's normal operating temperature just change and get lower?
My 1986 190E 2.3-16 has consumed two thermostats, with the symptom being it would warm up to its normal temp, about 82 degrees C, and then drop to about 70 or lower. Each lasted about 85,000 miles. And, at that temperature, mileage did suffer.
Based on my experience your temps seem pretty close to normal, but the real test is "did it change recently and abruptly?"
Good luck, Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)