The trailer frame certainly can be the 12V negative return, as for a car. It's also reasonable to run all the returns to the 12V panel (or some other convenient place) and thence back to the battery. In any case, the 12V negative should be bonded to the trailer frame, even if it's not the main return path.
Typical 12V wiring would be a circuit breaker or fuse close to the battery positive connection (within 18" is the standard, I think), then a relatively heavy connection to the 12V distribution panel via a disconnect switch. Most often the converter is also connected at the distribution panel, but it's okay and not uncommon to connect it elsewhere if that seems more convenient. From the 12V distribution panel, fuses protect the individual circuits.
There are a few things that have dedicated connections to the battery or to a buss bar or similar in that vicinity. Any unusually high draw device, such as a generator starter or sizable inverter, would not go through the distribution panel. The breakaway brake system for the trailer should also be wired directly so it can't be accidentally disabled.
