It would REALLY!!!! help to know the year of your SEC. I have both a 560 SEC and a Euro 500 SEC and they both will do approx 400. miles or darn close per tank of gas.
I think you need to take the car to a trusted Indy and do a Post Purchase Inspection. Money well spent. instead of GUESSING what it wrong and then throwing dollars at it. You will end up spending way more that the hour or so fee for the PPI, and besides you will have a solid baseline to base the car on.

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1986 Euro 500SEC "RUF" 9:1CR, "Rose"
1985 Euro 500SEC Cabriolet AMG Widebody
1982 Euro 500SEL "Blue"
2001 Texas Heeler (Aussie/Queensland X) "Sulphie",
2012 Queensland Red Heeler "Squeak"
Best dogs I've ever had.