Douglas Georgia is the place - there are at least 10 manufacturers there ... but those trailers are cheap for a reason.
You can search for enclosed or cargo trailers Douglas Georgia and they all come up and a few brokers too with sometimes better prices than direct
8K trailer are screwed together .032 side walls 24" on center and 1/2 OSB floors 4" or 5" .090 C Channel frames and the "birdcage is usually .045 1x2" rectangular tubing, some are even C channel wall framing, and Chinese 3500Lb axles. The rear doors are not usually rated over 2000 lbs either.
One of my buddies used to do that X-country trip for his job monthly before the Pandemic and he would bring back a trailer and sell it here and make 1000-1500 but he was getting them for 7000 OTD, not sure those deals are around.. Try Outlaw trailer thats where he got them from. If I remember right the 20' were cheaper than the 18' maybe its because they make more of them
When you get into the 11K range you get a nicer trailer, its not a Universal or CNC but they won't fall apart in 3-5 years ...