Hello Al! I'm not an expert electrician (so somebody else chime in to confirm), but here's what I've learned about wiring through research and experience ...
I assume you're wanting to install a 110-volt power inlet receptacle in your cargo trailer? Something like a ParkPower RV inlet? And then run power to all your outlets and fixtures?
That steel-wrapped wiring you're talking about is Armored Cable (AC) wire. You've got different types. With some, you use the sheathing as the ground; others, you have a separate conductor, things like that. They're designed for wet (sometimes exterior) locations and should generally be used with metal junction boxes and cable clamps. It's overkill for a camper interior so long as you keep it dry.
I prefer standard vinyl-insulated Romex cable. Much cheaper, easier to work with, safe, can be used with plastic junction boxes, etc. If you plan to run it underneath your trailer, you can always snake it through conduit. If you're concerned about interior moisture, you can snake the Romex through split-loom tubing, which you can buy $20 for a 100' roll.
Most folks use
14/2 w/ G solid copper wiring (my favorite) just like what you'd use in a house. If you plan to run big appliances, you can upgrade to 12/2 w/ G solid wiring. If you really want the flexibility of stranded wire, you can use 2-conductor THHN wire, but then you have to supply your own grounding conductor.
Hope that helps! If it doesn't, let me know