Grant, Chris, you should use the body as the negative return where possible on the DC side. Why? It is the biggest conductor and enables you to omit fuses in the negative and only fuse the positive. If you’ve fused both positive and negative, ignore the above. Why would both need to be fused? Well, take the scenario that your negatives that all run back to the battery terminal are all of differing sizes and the smallest somehow becomes trapped against the body of the vehicle/trailer and makes a connection. Will you know about it? The light on the end of it still works and no fuses have blown so no, you won’t. Further down the track your cutlery drawer collapses and traps the largest cable connected to the + of the battery through a 60A fuse. A short circuit is now made from this cable, through the body to the tiny lighting cable and then through that to the - battery terminal. The large cable and fuse see a load but the resistance keeps the load below 60A but applies close to 60A to the lighting wire, rated at 10A max which begins to get hot along its entire length, first compromising then fully melting the insulation and potentially setting fire to anything flammable it touches but no fuse blows. If the body of the vehicle/trailer is at least connected with a cable equal to the heaviest cable from the + side, this scenario won’t happen as the 60A fuse will immediately blow. At this point there is gain to be had by using the metalwork now solidly connected to the - terminal as the return as it is very low resistance and will improve conductivity by decreasing the overall resistance of the circuit.
On the 230v side, the frame must be connected to earth to satisfy electrical regulations (BS 7671 and BS 7909 temporary installations). Reasons are similar to the above and have no problem being mixed as they should not make a circuit through one another, mainly because the body is connected to - and to earth.
As a stand alone unit, the trailer for example using an inverter is classed as a generator. If it also has a mains ‘shore power’ connection then all earth points must be connected to chassis. An earth stake or mat would be beneficial if not essential. Why? In the instance without such stake or mat, your good lady is using the hairdryer and drops it in the puddle created by the outdoor shower. While standing on the wet ground and touching the trailer she completes the circuit back to the earthed trailer and will receive a shock.
You are of course using an RCD which will trip. But will it? Many generators, particularly small ones, only Earth the earth pin to the frame of the generator (giving two phases and an isolated ‘frame’ connection) and not to one side of the winding thus turning that phase into a neutral to go with the remaining phase and now an earth. Without the connection, the RCD will not trip as no fault circuit is made to bypass it (which is how it works).
The only way to avoid connecting the frame either to negative or to ground is to ensure under all conditions that the frame cannot possibly come into contact with any part of the circuit. And that is very difficult if not impossible on a metal structure such as a trailer or vehicle, subject to serious vibration. Grommets alone will often not be enough. I’ve seen them wear through with movement.
This would be along the lines of double insulation and along with regular inspection could be achieved but would rely on testing and inspection to ensure the fault connection was not made. Not recommended except under close supervision.
Chris, the only requirement is that the installer be ‘competent’.