Vary a lot from state to state, AND it can be difficult to get a straight answer from your state's employees.
When I bought a kit trailer in 2011, my (former) state of Colorado had passed new laws that required all "homemade" trailers--which really meant any trailer not purchased from a trailer dealer--to go through a safety inspection plus paperwork check, scheduled only for limited hours and at only one location. The county's registrar gave me a temporary license plate good for only 48 hrs. In that 48 hrs, I had to get the trailer inspected or else re-start the entire registration process. The inspector merely glanced at my rig as I drove in, then spent the entire time making sure I hadn't stolen the trailer or bought a stolen one. There was NO safety inspection whatsoever. Next, he voided the manufacturer-assigned VIN and said I had to permanently affix a metal plate with Colorado's own trailer VIN system number on it. (My trailer manufacturer says altering the metal VOIDS THE WARRANTY, so I did not attach the plate; I just carry the tag around with my registration papers and proof of insurance.) Only then was I allowed to obtain a regular license plate.
What really burns me up is that because the Colorado-assigned VIN was on the registration papers, when I moved to Washington, the registrar in my new county had to use Colorado's number despite the fact that I had left the manufacturer's VIN on the trailer. I still carry that damned metal tag around instead of being able to chuck it along with Colorado's stupid trailer-law mess.
Oh, yeah, Colorado's trailer registration fee went from under $20 for my trailer, to something like $150 for that first year.
Sounds like Florida is treating your salvaged trailer as "home-made" also. Do they not have any way for you to prove it is highway-worthy?