I was pulling a little 13-foot Serro Scotty (9-foot interior space) with 3.4 V6 and getting 13-14 mpg fully loaded with 2 kids and enough food and camping gear for a month of boondocking. We kept track of almost every tank on a 7000 mile trip, pretty consistent mpg but elevation made a noticeable difference over 4000 feet. The one time we found Ethanol-free non-premium gas we were getting 14.5 or so.
Granted the trailer supposedly only weighed 900 lbs dry, it did have the frontal profile of a typical old aluminum trailer and from everything I've read, aerodynamics plays a bigger role than weight in towing-ability so long as the vehicle is comfortably within spec and you're not going uphill. So if I end up with a cargo trailer, if I can squeeze better drag coefficient out of it I would expect better tow ability even if it weighs 500 - 1000 pounds more.
The trailer tails on semis get 5.5% better fuel economy at 65mph. Sounds great but I don't know if they make anything for a cargo trailer since that's where the tailgate drops down.
I'm pretty sure a smooth bump like NoseCone placed where the front of the trailer rises above the TV would squeeze better mpg. 5th wheel towers seem to have surprisingly good mpg for their weight and for how gigantic the frontal area is. I think it's because their's no gap between TV and trailer and the trailer itself is slippery and round.
I usually have a roof box on the TV for extra cargo. I'm thinking of getting a full-width one and modding it with something like the Airtabs mentioned by another poster to help deflect air over the trailer.
Popup camper not an option, bears, winters, and serious durability concerns. I'll go buy a diesel first. I remember winters in Texas in a popup; when we were waking up to find ice on the interior of our camper we upgraded to a 50s era canned ham, which was probably cheaper actually. Still no heater but with 6 warm bodies and a sliver of insulation it kept us pretty warm.