You can connect all (-) phases of DC and the AC ground to the same point on the trailer's chassis (frame) or connect the DC (-) to any point on the frame and the AC ground to any point on the frame. You just want the battery (-), the converter's DC output (-), the 12V distribution panel (-), and the AC load center ground bus all connected to the frame of the trailer in some manner.
MO is that multiple ground points can lead to separate system failures. A single ground connection, prepared so that any wiring is in good contact with frame steel and protected with an anti-corrosion material is the best choice. If proceeding this way, drill and tap a hole for fastener. Use a serrated locking washer and flat washers between wire terminals. This allows for a secure connection. Alternatively, a good. larger, self tapping bolt can also be used.