It's all about wind resistance. The two ways to improve fuel economay with a TT are to choose a TT that catches less wind, and/or to drive more slowly.
Let's say you plan to drive 60 mph. From what I've experienced and read, most boxy trailers seem to typically result in 10-11 mpg. 8' wide TTs probably 10, and 7' wide about 11. Going to a very rounded trailer (molded fiberglass like Scamp, Casita, Escape or aluminum like Airstream) are likely to give a boost to about 14-16 mpg. Going to a popup (including non-tent popups like Aliner, Chalet, Hi-Lo, Trailmanor) could yield a bit more, perhaps 16-18 mpg, because they sit lower and catch less wind.
Going longer or heavier generally have less effect than going larger on frontal area.
Escape Trailers of Chilliwack BC are very high quality, with a 19' and soon a 21' model. Oliver Trailers in TN is starting back up with a 22' molded FG trailer, and they are also known for top quality with plenty of bells and whistles. Li'l Snoozy in SC makes a 17' trailer that looks like a clam shell and tows easily. Airstream has a name for beautiful, luxurious trailers, although some feel their quality is not what it used to be... and they have all those joints and rivets on the outer surface. If your goal is to optimize fuel economy but you don't want a folding trailer, these are some of the best options.
If you did spend 6 months in the unit, I doubt you'd want a popup; you'd lose the upper storage cabinets on almost any of them, and they would tend to be more drafty.
I tow all the time with my Highlander for both work and play. Usually about 11 mpg. Lately I have been wishing for either less frontal area or more engine, as it seemed to struggle more this summer through the mountains and the transmission was getting hotter than I like. In my experience the HL will handle a TT of 3000-3500 lbs or so, and 16' or 17', but you have to figure on about 60 mph tops and a fair bit slower (with upshifts) on the hills. I'm dreaming of a Grand Cherokee diesel or maybe an Explorer 3.5 ecoboost.
