Not bad Roostre, is that a 7x16 Haulmark?
I'm the crazy towing fool around these parts, right now I'm driving around Savannah, Ga yanking a 75kw Cat Diesel generator to film set locations. 6900 lbs is the trailer's GVWR, and I know from a previous experience that this setup is just about 6800 when fully fueled. Add to that, probably close to 750 lbs of FEEDER cables (100 foot of 4/0 gauge cable, times 5 conductors = 500 ft of cables) and this is a BIG load to be hauling anywhere.
The CRD has been handling it just fine. Drinking the diesel, certainly, but not a big problem on the haulage. I also have pulled a 7.5x20 enclosed box like the one you picture, only bigger. That scaled at just about 2000 empty, and 7000 fully loaded, with a scary 1200 lbs tongue. I've loaded and re-loaded that trailer with the heaviest things way behind the axle, and still thats where it ends up. Bad design, but I don't move it that often so I don't care. Here's an old picture of the 70kw generator that I scaled to know it was 6840 fueled... This genset is just about the same unit.

For the original poster, I've towed U-haul trailers too, and from experience they are WAY WAY overbuilt. A 6x12 Uhaul will weigh EMPTY as much as my 7.5x20. The NORMAL weight for a 6x12 (like my other trailer, a 6x12 Haulmark) is like 1500# or so. You get the same loading capacity if you have the same number of axles, a dual-axle would offer at least 2500# of carrying, single would be 1800# or so. U-Hauls are double-axle because they have to plan on customers filling the trailer completely with cement - REALLY overloading it, and yet would still be safe. This overbuilding costs you money and fuel. You would be MUCH better served (if you can afford it) to look on your local craigslist or on crazedlist.org (searches multiple Craigslist sites) for a used trailer and buy that. Use it for your move, then toss it back on CL and get your money back out of it. I did that and got a 6x12 Haulmark for cheap (actually the guy I bought it from doubled his own money after his move!)
If you put anything into the trailer like upgrades, tiedowns, interior rails, lights or new stuff / repairs / tires... Just add that to the sale price and you will get it all back. Most states will register and plate a trailer for like $40 or so, but I've also never been hassled by not plating a trailer if you have a bill of sale that shows you've owned it less than 30 days and are moving to a new state anyway. Then you can "accidentally" forget to tag it while you sell it yourself, and let the new owner file the title and tag. Saves even more money.
You may find that you want to keep the trailer around tho as a convenient portable shed / workshop, or even use that instead of your CRD's interior when moving stuff around. As far as mileage / handling, the 6x12 is a perfect size for the CRD, or even a 6x14. I prefer double axle for safety and capacity, and having a brake controller will make your life happier and safer. You should get a solid 19mpg towing at highway speeds, the trick is to remember that you are pulling something heavy. It isn't the top speed that will eat your mileage, it is the acceleration. DON'T BEAT ON IT.