Separate from ALL other electrical shenanigans, at least minimal lighting with solar/small battery. DIY or kits. You will ALWAYS need lighting and it doesn't take a lot.
Tie down points? E track. Very flexible system, lots of accessories, the straps cost the same as any other strap. Plus there's various hooks and hangers you can clip in to hold stuff directly. So there is a "buy in" to the system, but in a trailer that size, it's a good system, and need not be an huge expense. You can get "industrial" brands of stuff for lots of money, but unless you're doing LTL deliveries for a living, you're not gonna wear out a Harbor Freight strap and ratchet. Talk to the trailer manufacturer as to how it is installed on THEIR trailer walls and THEIR floors. Drywall screws won't cut it obviously, but they might want proper wood screws, they may want rivets from the outside.... The floor will be different from the walls... Are you tying to the walls, tying to the floor for wheels.... That's the sort of thing that they charge DEARLY for. To do it takes hours to get the first one, but after that, all of those ten million little fasteners go quick.
What else are you hoping to do with this? "Camping trailer up north? Are you gonna put bunks in it? Or just carry stuff? Personally (it's use case, it's YOUR use, so I can't tell you what to do), but if it were me, aside from those two "basics", if you wanted "real" power inside, I'd do that 100 percent separate from the solar LED "basic functional lighting". A power receptacle for whatever generator you already own, a small load center "somewhere that suits you", probably and hopefully right close to where the receptacle engers, and whatever "real electrical stuff" you want to run for your storage, mobile machine shop, bitcoin mining operation, or mobile command center can be done to suit with (mostly) standard box store electrical shenanigans.
As was already said, it's hard to offer "exact" solutions to a problem that's loosely defined. There's a LOT of options.
