Hindsight being what is....and all. By the time you've taken all steps backward to retro-fit a pre-manufactured CT with the stuff like Insulation, wiring, windows, etc., etc., you could actually come out ahead starting out with just a flatbed frame.
Wall and ceiling struts, roof and wall skins and some hinges is really all your getting. But, then you are forced to compromise and work around things like wall and roof cavity space for insulation, pre determined interior width and height, overall shape and size, placement of frame and support members, poor quality factory workmanship, design and materials, etc.
On the other hand, few of us posses all the design, engineering and production skills and facilities to start from scratch, and end up with a quality, purpose built unit in a timely, affordable manner. If we did, we wouldn't be having this conversation. I believe the phrase "Jack of all trades, master of none" applies well here. So the process has to be it's own reward. You will learn something, and grow and evolve as a person, hopefully anyways.
You start with a chunk of coal. But with enough time and pressure, and cutting and polishing you end up with a diamond.
If you look close enough there will still be flaws, but overall, brilliant and dazzling with a value that is beyond it's weight as a commodity. And you will have a pretty nice trailer, as well.
No retreat, No surrender
McDave