In the past I have seen some Subaru owners on the fiberglassrv forum who tow Scamps, Casitas, and similar molded fiberglass trailers. They tend to have a relatively small frontal area; the smooth sides & rounded corners help keep wind resistance down. Many of them get 16-19 mpg while towing. I hope that helps.
If at first you don't succeed, skydiving is not for you.
19' ETI Escape trailer