It's always worth paying attention to what slowmover has to say.
OTOH, minimizing the mileage loss is setting your aspiration low. Although they don't mirror your situation, I can think of two examples where the mileage went up!
Boat tail project based on a single wheel trailer...
Compact camper alternative: New Beetle TDI with hard mounted pop up tent
One is a single wheeled trailer, the other a repositioned car-topper. In your case I could see a trailer that has a dual-single axle with castering wheels less than 18" apart and two clamps on the tow vehicle. The wheels could be at the end of a Tropfenwagen-style tail with a 6' high 45° edge.
Think big. The only question would be where you put the door and how high your mileage can be.