I would agree the aero load on the sedan due to the shape of the trailer is a likely source of the added fuel consumption. I drove from CT to TN recently to pick up a jet ski with the 240D. This car has about 320,000 miles on it and runs pretty strong. We got 32 mpg on the way down, without a trailer. We picked up the jet ski and its trailor there, total wt. of about 900 lbs, and got 28.5 mpg on the way back. The jet ski was pretty low and tightly tucked in behind the car, so I think the aero effect was minimized. In addtion, in deference to the car's age and mileage, and tendency to spew oil at high speeds, we drove under 70 mph the whole way down and back, with the way back generally held to 65 mph. I was pleasantly surprised. Getting about the same result driving it from CT to Upstate NY (Willsboro on Lake Champlain), which includes some good, long, steep hills on the Northway. Jim

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Own:
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

Owned:
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)