Here is one of the few examples for a trailer with a reported coefficient of aero. drag.
It's a 1988 Voyager 2000,from C.I.Caravans Ltd.,The Oaks,Fordham Rd.,Newmarket,Suffolk CB8 7AL,England. (Re. Popular Science Magazine,July,1988,pg 92.
They're claiming Cd 0.23.No mention of whether that's by itself,or behind a tow vehicle.