States vary, not much else we can say
I bought an antique Uhaul trailer axle for $25 on a whim thinking I could build a trailer with it (what a concept eh?) but it sat by the barn another 2-3 years. Then after a work injury my life took a tumble and my world came crashing down.
It wasn't easy with a bunged up back and shoulder but my father was getting in bad shape and I just decided it was time to go home so I finally started building that trailer.
It took awhile, and a lot of new words but that trailer finally got built, and in TN they had titles. How could I ever manage that(it took 6 months to find TN trailer requirements, lights placing, tow chain specs etc etc etc) but finally learned how.
It cost $25 and I had to be there when the inspector arrived so I just watched and waited. Woke up one morning to a TN Highway Patrol vehicle sitting in my driveway, got out there and he was Hammering(loudly!) on the trailer tongue (testing the welds?)
When I got there saw he had a set of type punches in a box and was driving a serial number onto the trailer. I signed a paper after checking the boxes, filling in the blanks (one asked if I had any reason to believe the trailer was not safe, which I checked anyway, knowing it was not).
He handed me a copy and left, and in a couple weeks I had a title in the mailbox, in my name, and in the box "type" it said HMD
The reason I felt it unsafe was the old Uhaul had a drop axle(not straight end to end) and just outside the leaf springs turned upwards about 4 inches and if I'd had a blowout the Ubolts would be digging into the pavement at 70 mph.
I had a new axle built(straight) for $150 and new wheels , another $150, and it was good to go

Funny that, I'd built a trailer around an axle, then changed the axle
After getting to OK and time to change my tags and titles the OK title office would NOT change my trailer title. I was proud of that little trailer and wanted my title!! haha, nope! OK doesn't use trailer titles, in OK they just use your truck tag #, which really makes no sense to me to this day
A few old shots of it (I was halfway through before starting to take them :/ )