A big thing to keep in mind is frontal area, you can get great Cd but if it has the frontal area of a standard camper (mine is almost 10 feet tall and 8 feet wide or 80 sqft) you still will be at a disadvantage compared to say a pop-up camper that is just a box but follows almost entirely in the wake of the tow vehicle. Best of all worlds would be a low profile pop-up camper that actually improves the Cd of the tow vehicle by making basically a boat tail trailer.
Heavy isn't necessarily a problem if you are talking about long steady runs without need for braking. You burn more gas getting weight up to speed but once at speed it's just stored potential energy. Just don't waste it in the brakes and it will return to you at some point. So when going up hills, let speed go so when you crest the hill you can just roll down without brakes. A mountain pass will be too long and you will have to brake probably, but on the little hills you can build speed before, lose speed on, and then coast all the way down and then weight isn't such a big penalty. certainly not as big as aerodynamics. Again best of all worlds would be light, aerodynamic, and no bigger frontal area than the tow vehicle.
This was my first attempt on a $1000 Hi-lo I picked up that had roof damage anyway. I thought I was just going to use the appliances and the frame but it turned out fixing he roof wasn't that hard so I fixed it, and then messed around with the basics. I added a boat tail that also worked as a storage box on the back. Turned out it may have helped aero but wasn't useful as extra storage, as anything of weight back there took too much weight off the tongue and caused sway. I also added some fairings on the front and on the awning. It did do pretty good, that SUV on it's best day would only get 18-19 mpg and towing that trailer we still would see 14-15 mpg. Towing my current camper, a standard box, that Aspen never got better than 9 mpg. I never towed the Hi-lo with my Cummins but it could do over 20 mpg unloaded and towed the new box camper at 12-13 mpg. I have a feeling it may have hardly noticed this Hi-lo. Now the Hi-lo, the Cummins, and the Aspen are all gone and we have a 2500 Suburban to tow the big box which it hasn't actually done yet so I don't know what kind of MPG it will get.
I have another plan to build my own off road Australian style pop-up trailer. One where the top hinges over and becomes the floor of the stand up tent area, the kitchen pulls out from under the bed into the stand up area, and the bed is the footprint of the original 5x8 trailer, just built 2' off the deck. The tent would all be attached and fold out like a pop out book does when you open it. I have foam model built but can't find the pictures now.
basically like this design
here is a video for more clarity about 3:36 into it