We are all wishing for better gas mileage but to sacrifice storage and put yourself in cramped spaces just seems foolish to me. The real issue with any travel trailer is weight and design. I lucked out and stumbled upon a used R-Vision Trail Light Travel Trailer when looking for a unit that would fit my needs. Unlike most Travel Trailers this unit rode lower to the ground, had a sealed bottom (less wind drag), and had a light weight metal frame. The 26 foot unit I found had fantastic storage spaces every feature anyone would want and only weighed 3370 Lbs (dry weight). It tows so well I forget it is back there for the trip!!! Using a Toyata FJ Cruiser SUV, V6 powered I get between 14 and 16 MPG. Sad thing is this maker got out of the TT business in early 2014 so the only source for a Trail Light TT is on the used market.
Only way to find better fuel mileage is to find a well designed, light weight unit....back in the 1980s I owned a few Sunline Travel Trailers but they too are long gone. I'm sure there must be another light weight TT out there that doesn't utilize the extreme sloping roof line I see in the IPod units. Such a roof line might work well if you were a dwarf. (IMHO)