Food for thought: Cargo trailers are designed to protect the inside from the weather and dust with minimal aesthetics and no insulation. The framing is not designed to provide a base to mount insulated and aesthetic interior cladding to. You can certainly do it by doing a van life hackjob and just mount pallet wood to the interior framing members after slapping in some R zero shiny bubble wrap and it will probably work ok for leisurely urban driving. Or you could build in a complicated framing system and do it a little better at the cost of some interior space. But if you plan on leaving the pavement, you will probably have long term problems because cargo trailers are super flimsy on bumpy roads, and a bunch of stuff mounted shoddily to the interior will probably rattle apart after a while.
Not to mention, cladding over sheetmetal exterior walls with anything other than a complete sprayfoam job on the inside of the sheetmetal, then camping anywhere where the temperature inside is warmer than the exterior temp enough to cause condensation, is just asking for long term mold/fastener rust issues unless you are able to vent behind the cladding enough to consistently dry out the condensation.
If I were using a cargo trailer for a tent, I would probably find a cheep or discounted vinyl flooring product that can be adhered to the deck, and for the walls I would leave them be other than mounting (securely) a few cabinets, countertop/sink/bed platform/etc. You could build a "floor pack" with 3/4" or 2" framing on the floor, fill the voids with rigid sheet foam and gaps with cans of spray foam, then sheet over it with 1/4 ply and glue down vinyl over that. The problem is that an insulated floor is not going to do a lot with uninsulated walls/ceiling.
Then you get to the ceiling. Most cargos have some sort of arch in the ceiling, which also illustrates the minimal wall framing. The arch squeezes enough strength out of the minimal wall framing to resist snow load sufficiently. If the walls were stout, the arch would not be needed. But the arch severely limits how you can clad the ceiling as you either need to frame in a square drop ceiling (suspended from the flexible, minimal framing members) or uses a flexible cladding material that can bend to the arch of the ceiling.
Don't mean to rain on your parade, and I've never converted a cargo trailer to a camper, but I do have a decent cargo trailer that I purchased new. Did two loads in it on our 1000 mile move a few years ago. The first load was mostly heavy stuff, including numerous tools and a 700# safe with 300#s in it. I installed tie down hardware on the interior walls and used tie down anchors and shelf brackets to make an extensive system to keep everything secure, including building a large cargo shelf around the safe once it was loaded. Cargo straps designed to mount to the cargo rails were used as well. As soon as you put any tension on the straps, you can see the whole wall/ceiling structure racking. This is fine for transporting cargo, but it is not a stable base for finishing out a camper space, in my opinion.
More food for thought: When you see people on social media/youtube going through their "builds" for van life or cargo campers, try to find one where they do an update after 1 year of heavy use and are honest about the drawbacks of their DIY project. Most of them will have "upgraded" to a different rig for unknown reasons, on others you will see their DIY rig suddenly disappear from the channel. Occasionally you will see one of the more honest ones go through all the bad things about their build, but that is rare. The problem here is that the content creators get positive reinforcement for their build through channel growth, sponsorships, thumbs-up dopamine hits, etc. Then they spend the next few months putting out content extolling the virtues of what they did, riding that dopamine wave. They answer questions from viewers who are so inspired by them and are now spending their own time and money to do the same things because look how great it is. After months of this, if they start having problems or realizing that their build sucks, they will never talk about it because of the feared backlash.