This sounds like an interesting project. There have been a few folks that have posted on this forum about similar but most of them can't get past the planning stages. One gal had some rather grandiose plans with a cargo trailer but more or less lost her mind when the suggestions or criticism got a bit tough. Glad to hear someone has the tenacity to follow through.
FWIW, I gutted and restored a trailer a couple of years ago. The restoration thread linked in my sig line, The Cowboy/Hilton, has pictures and information about wiring and such. I chose to install 50 amp service but the basic wiring process is identical for 120V and 12V schemes. This Forum got me interested in solar power for RV's and I ended up with a solar charging system. It is invaluable if you camp off-grid, which I choose to do almost exclusively.
Best of luck with your wiring and the cargo outfit. Keep us posted and put up some pictures, if you have the chance.